MIPS/Linux/native: Supply $zero for the !PTRACE_GETREGS case

MIPS/Linux/native: Supply $zero for the !PTRACE_GETREGS case

With native MIPS/Linux targets the $zero register is inaccessible, with
its supposed context slot provided by the OS occupied by the $restart
register.  The PTRACE_GETREGS path takes care of it by artificially
supplying the hardwired contents of $zero in `mips_supply_gregset' or
`mips64_supply_gregset', as applicable, however the PTRACE_PEEKUSER
fallback does not, making the register unavailable, e.g.:

(gdb) info registers
         zero       at       v0       v1       a0       a1       a2       a3
R0    <unavl> 00000001 00000001 d2f1a9fc 00000000 00000000 00417158 00417150
           t0       t1       t2       t3       t4       t5       t6       t7
R8   00000004 00000000 fffffff8 00000000 00000000 00000000 00000001 00000007
           s0       s1       s2       s3       s4       s5       s6       s7
R16  00000000 00405e30 00000000 00500000 00000000 0052ec08 00000000 00000000
           t8       t9       k0       k1       gp       sp       s8       ra
R24  00000000 00417008 00000000 00000000 0041e220 7fff4ce0 7fff4ce0 00405d0c
       status       lo       hi badvaddr    cause       pc
      <unavl> 00441cf1 00000017 00417004 00800024 00405d10
         fcsr      fir  restart
     00800000 00f30000 00000000
(gdb)

or (under certain circumstances):

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

This is specifically because `mips_linux_register_addr' and
`mips64_linux_register_addr', both correctly return -1 for
MIPS_ZERO_REGNUM, and therefore `linux_nat_trad_target::fetch_registers'
faithfully marks this register as unavailable.

Supply this register artificially then in the PTRACE_PEEKUSER case as
well, correcting this issue.

	gdb/
	* mips-linux-nat.c (mips_linux_nat_target::fetch_registers):
	Supply the MIPS_ZERO_REGNUM register.