Process exit status is leader exit status testcase

This adds a multi-threaded testcase that has all threads in the
process exit with a different exit code, and ensures that GDB reports
the thread group leader's exit status as the whole-process exit
status.  Before this set of patches, this would randomly report the
exit code of some other thread, and thus fail.

Tested on Linux-x86_64, native and gdbserver.

Co-Authored-By: Pedro Alves <pedro@palves.net>
Change-Id: I30cba2ff4576fb01b5169cc72667f3268d919557