[gdb/tdep, aarch64] Fix frame address of last insn

[gdb/tdep, aarch64] Fix frame address of last insn

Consider the test-case test.c, compiled without debug info:
...
void
foo (const char *s)
{
}

int
main (void)
{
  foo ("foo");
  return 0;
}
...

Disassembly of foo:
...
0000000000400564 <foo>:
  400564:       d10043ff        sub     sp, sp, #0x10
  400568:       f90007e0        str     x0, [sp, #8]
  40056c:       d503201f        nop
  400570:       910043ff        add     sp, sp, #0x10
  400574:       d65f03c0        ret
...

Now, let's do "info frame" at each insn in foo, as well as printing $sp
and $x29 (and strip the output of info frame to the first line, for brevity):
...
$ gdb -q a.out
Reading symbols from a.out...
(gdb) b *foo
Breakpoint 1 at 0x400564
(gdb) r
Starting program: a.out

Breakpoint 1, 0x0000000000400564 in foo ()
(gdb) display /x $sp
1: /x $sp = 0xfffffffff3a0
(gdb) display /x $x29
2: /x $x29 = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3a0:
(gdb) si
0x0000000000400568 in foo ()
1: /x $sp = 0xfffffffff390
2: /x $x29 = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3a0:
(gdb) si
0x000000000040056c in foo ()
1: /x $sp = 0xfffffffff390
2: /x $x29 = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3a0:
(gdb) si
0x0000000000400570 in foo ()
1: /x $sp = 0xfffffffff390
2: /x $x29 = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3a0:
(gdb) si
0x0000000000400574 in foo ()
1: /x $sp = 0xfffffffff3a0
2: /x $x29 = 0xfffffffff3a0
(gdb) info frame
Stack level 0, frame at 0xfffffffff3b0:
 pc = 0x400574 in foo; saved pc = 0x40058c
(gdb) si
0x000000000040058c in main ()
1: /x $sp = 0xfffffffff3a0
2: /x $x29 = 0xfffffffff3a0
...

The "frame at" bit lists 0xfffffffff3a0 except at the last insn, where it
lists 0xfffffffff3b0.

The frame address is calculated here in aarch64_make_prologue_cache_1:
...
  unwound_fp = get_frame_register_unsigned (this_frame, cache->framereg);
  if (unwound_fp == 0)
    return;

  cache->prev_sp = unwound_fp + cache->framesize;
...

For insns after the prologue, we have cache->framereg == sp and
cache->framesize == 16, so unwound_fp + cache->framesize gives the wrong
answer once sp has been restored to entry value by the before-last insn.

Fix this by detecting the situation that the sp has been restored.

This fixes PRs tdep/30010 and tdep/30011.

This also fixes the aarch64 FAILs in gdb.reverse/solib-precsave.exp and
gdb.reverse/solib-reverse.exp I reported in PR gdb/PR29721.

Tested on aarch64-linux.
PR tdep/30010
PR tdep/30011
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30010
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30011