[lldb][AArch64] Account for extra libc frames in PAC unwind test

Running this on Amazon Ubuntu the final backtrace is:
```
(lldb) thread backtrace
* thread #1, name = 'a.out', stop reason = breakpoint 1.1
  * frame #0: 0x0000aaaaaaaa07d0 a.out`func_c at main.c:10:3
    frame #1: 0x0000aaaaaaaa07c4 a.out`func_b at main.c:14:3
    frame #2: 0x0000aaaaaaaa07b4 a.out`func_a at main.c:18:3
    frame #3: 0x0000aaaaaaaa07a4 a.out`main(argc=<unavailable>, argv=<unavailable>) at main.c:22:3
    frame #4: 0x0000fffff7b373fc libc.so.6`___lldb_unnamed_symbol2962 + 108
    frame #5: 0x0000fffff7b374cc libc.so.6`__libc_start_main + 152
    frame #6: 0x0000aaaaaaaa06b0 a.out`_start + 48
```
This causes the test to fail because of the extra ___lldb_unnamed_symbol2962 frame
(an inlined function?).

To fix this, strictly check all the frames in main.c then for the rest
just check we find __libc_start_main and _start in that order regardless
of other frames in between.

Reviewed By: omjavaid

Differential Revision: https://reviews.llvm.org/D154204