MIPS/gdbserver: Fix issues with $zero register reads

MIPS/gdbserver: Fix issues with $zero register reads

Consistently supply hardwired $zero as a zeroed register, correcting
issues with the PTRACE_GETREGS path that currently copies the value of
$restart into $zero as illustrated by this program:

$ cat read.c

int
main (void)
{
  char buf[1024];
  ssize_t size;

  size = read (0, buf, sizeof (buf));

  return size;
}
$

and this corresponding debug session:

(gdb) break main
Breakpoint 1 at 0x120000970: file read.c, line 9.
(gdb) target remote :2346
Remote debugging using :2346
Reading symbols from .../sysroot/mips-r2-hard/lib64/ld.so.1...done.
0x000000fff7fca5a0 in __start ()
   from .../sysroot/mips-r2-hard/lib64/ld.so.1
(gdb) continue
Continuing.

Breakpoint 1, main () at read.c:9
9	  size = read (0, buf, sizeof (buf));
(gdb) info registers
                  zero               at               v0               v1
 R0   0000000000000000 0000000000000001 000000fff7ffe710 0000000000000000
                    a0               a1               a2               a3
 R4   0000000000000001 000000ffffffeb88 000000ffffffeb98 0000000000000000
                    a4               a5               a6               a7
 R8   000000fff7fc8800 000000fff7fc38f0 000000ffffffeb80 2f2f2f2f2f2f2f2f
                    t0               t1               t2               t3
 R12  0000000000000437 0000000000000002 000000fff7ffd000 0000000120000a00
                    s0               s1               s2               s3
 R16  000000fff7fc7068 0000000120000b90 0000000000000000 0000000000000000
                    s4               s5               s6               s7
 R20  0000000000521d88 0000000000522608 0000000000000000 0000000000000000
                    t8               t9               k0               k1
 R24  0000000000000000 0000000120000970 0000000000000000 0000000000000000
                    gp               sp               s8               ra
 R28  000000fff7fc8800 000000ffffffea50 0000000000000000 000000fff7e4088c
                status               lo               hi         badvaddr
      0000000000109cf3 0000000000005ea5 0000000000000211 000000fff7eadf00
                 cause               pc
      0000000000800024 0000000120000970
                  fcsr              fir          restart
              00000000         00f30000 0000000000000000
(gdb) continue
Continuing.
^C

Program received signal SIGINT, Interrupt.
0x000000fff7f084ac in __GI___libc_read (fd=0, buf=0xffffffe640, nbytes=1024)
    at ../sysdeps/unix/sysv/linux/read.c:27
27	  return SYSCALL_CANCEL (read, fd, buf, nbytes);
(gdb) info registers
                  zero               at               v0               v1
 R0   0000000000001388 0000000000000001 0000000000000200 000000fff7ffe710
                    a0               a1               a2               a3
 R4   0000000000000000 000000ffffffe640 0000000000000400 0000000000000001
                    a4               a5               a6               a7
 R8   000000fff7fc8800 000000fff7fc38f0 000000ffffffeb80 2f2f2f2f2f2f2f2f
                    t0               t1               t2               t3
 R12  00000000000005e3 0000000000000002 000000fff7ffd000 000000012000099c
                    s0               s1               s2               s3
 R16  000000fff7fc7068 0000000120000b90 0000000000000000 0000000000000000
                    s4               s5               s6               s7
 R20  0000000000521d88 0000000000522608 0000000000000000 0000000000000000
                    t8               t9               k0               k1
 R24  0000000000000000 000000fff7f2da20 0000000000000000 0000000000000000
                    gp               sp               s8               ra
 R28  000000fff7fc8800 000000ffffffe600 0000000000000000 000000012000099c
                status               lo               hi         badvaddr
      0000000000109cf3 00000000000001e6 00000000000000be 000000fff7f08470
                 cause               pc
      0000000000800020 000000fff7f084ac
                  fcsr              fir          restart
              00000000         00f30000 0000000000001388
(gdb)

and with the PTRACE_PEEKUSR path that does not supply this register at
all, causing issues analogous to ones addressed for the native MIPS
backend with commit 4e6ff0e1b86f ("MIPS/Linux/native: Supply $zero for
the !PTRACE_GETREGS case"):

(gdb) info registers
                  zero               at               v0               v1
 R0      <unavailable> 0000000000000001 0000000000000001 0000000000000000
                    a0               a1               a2               a3
 R4   00000001200212b0 0000000000000000 0000000000000021 000000012001a260
                    a4               a5               a6               a7
 R8   000000012001a260 0000000000000004 800000010cab1680 fffffffffffffff8
                    t0               t1               t2               t3
 R12  0000000000000000 000000fff7edab68 0000000000000001 0000000000000000
                    s0               s1               s2               s3
 R16  000000fff7ee2068 0000000120008b80 0000000000000000 0000000000000000
                    s4               s5               s6               s7
 R20  000000000052e5c8 000000000052f008 0000000000000000 0000000000000000
                    t8               t9               k0               k1
 R24  0000000000000000 00000001200027c0 0000000000000000 0000000000000000
                    gp               sp               s8               ra
 R28  00000001200212b0 000000ffffffc880 000000ffffffc880 0000000120005ee8
                status               lo               hi         badvaddr
         <unavailable> 0000000000943efe 000000000000000e 000000012001a008
                 cause               pc
      0000000000800024 0000000120005ee8
                  fcsr              fir          restart
              0e800000         00f30000 0000000000000000
(gdb)

and (under certain circumstances):

(gdb) next
Register 0 is not available
(gdb)

The problem with PTRACE_GETREGS happens because `mips_store_gregset'
supplies the contents of register slot #0, occupied by $restart, to
$zero.  The problem with PTRACE_PEEKUSR happens because for $zero
`mips_cannot_fetch_register' returns one, and no alternative way to
supply that register has been defined.

Correct `mips_store_gregset' then for the PTRACE_GETREGS case and add
`mips_fetch_register' for the PTRACE_PEEKUSR case.

	gdb/gdbserver/
	* linux-mips-low.c (mips_fetch_register): New function.  Update
	preceding comment.
	(mips_store_gregset): Supply 0 rather than $restart for $zero.
	(the_low_target): Wire `mips_fetch_register'.