Implement gdbarch_stack_frame_destroyed_p for aarch64

Implement gdbarch_stack_frame_destroyed_p for aarch64

The internal AdaCore testsuite has a test that checks that an
out-of-scope watchpoint is deleted.  This fails on some aarch64
configurations, reporting an extra stop:

    (gdb) continue
    Continuing.

    Thread 3 hit Watchpoint 2: result

    Old value = 64
    New value = 0
    0x0000000040021648 in pck.get_val (seed=0, off_by_one=false) at [...]/pck.adb:13
    13	   end Get_Val;

I believe what is happening here is that the variable is stored at:

    <efa>   DW_AT_location    : 2 byte block: 91 7c 	(DW_OP_fbreg: -4)

and the extra stop is reported just before a return, when the ldp
instruction is executed:

   0x0000000040021644 <+204>:	ldp	x29, x30, [sp], #48
   0x0000000040021648 <+208>:	ret

This instruction modifies the frame base calculation, and so the test
picks up whatever memory is pointed to in the callee frame.

Implementing the gdbarch hook gdbarch_stack_frame_destroyed_p fixes
this problem.

As usual with this sort of patch, it has passed internal testing, but
I don't have a good way to try it with dejagnu.  So, I don't know
whether some existing test covers this.  I suspect there must be one,
but it's also worth noting that this test passes for aarch64 in some
configurations -- I don't know what causes one to fail and another to
succeed.