[gdb/tdep] Fix gdb.base/large-frame.exp for aarch64

[gdb/tdep] Fix gdb.base/large-frame.exp for aarch64

On aarch64, I run into:
...
FAIL: gdb.base/large-frame.exp: optimize=-O0: backtrace
...

The problem is that the architecture-specific prologue analyzer fails to
handle the first two insns in the prologue properly:
...
0000000000400610 <func>:
  400610:       d2880210        mov     x16, #0x4010
  400614:       cb3063ff        sub     sp, sp, x16
  400618:       a9007bfd        stp     x29, x30, [sp]
  40061c:       910003fd        mov     x29, sp
  400620:       910043a0        add     x0, x29, #0x10
  400624:       97fffff0        bl      4005e4 <blah>
...
so we get:
...
$ gdb -q -batch ./outputs/gdb.base/large-frame/large-frame-O0 -ex "b func"
Breakpoint 1 at 0x400614
...

Fix this by:
- fixing the support for the first insn to extract the immediate operand, and
- adding support for the second insn,
such that we have:
...
Breakpoint 1 at 0x400624
...
Note that we're overshooting by one insn (0x400620 is the first insn after the
prologue), but that's a pre-existing problem.

Tested on aarch64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29408