gdb/testsuite: add test for backtracing for threaded inferiors from a corefile

gdb/testsuite: add test for backtracing for threaded inferiors from a corefile

This patch is based on an out-of-tree patch that fedora has been
carrying for a while. It tests if GDB is able to properly unwind a
threaded program in the following situations:
* regular threads
* in a signal handler
* in a signal handler executing on an alternate stack

And the final frame can either be in a syscall or in an infinite loop.

The test works by running the inferior until a crash to generate a
corefile, or until right before the crash. Then applies a backtrace to
all threads to see if any frame can't be identified, and the order of
the threads in GDB. Finally, it goes thread by thread and tries to
collect a large part of the backtrace, to confirm that everything is
being unwound correctly.

Co-Authored-By: Andrew Burgess <aburgess@redhat.com>
Reviewed-By:  Luis Machado  <luis.machado@arm.com>
Approved-By: Luis Machado <luis.machado@arm.com>