Add frame pointer unwinding for aarch64 and relax backtrace testcases.

Add frame pointer unwinding for aarch64 and relax backtrace testcases.

If we don't find any debug information for a given frame, we usually
cannot unwind any further. However, the binary in question might have
been compiled with frame pointers, in which case we can look up the
well known frame pointer locations in the stack snapshot and use them
to bridge the frames without debug information.

Relax the backtrace core testcases a little by allowing a duplicate
sigusr2 frame or a backtrace ending with an invalid register. Both of
which can happen if the frame pointer unwinder guesses slightly wrong.

Signed-off-by: Ulf Hermann <ulf.hermann@qt.io>
Signed-off-by: Mark Wielaard <mark@klomp.org>