gdb_monitor: improve the backtrace test

With this patch, functions which execute blocking syscalls on Fiasco.OC
are built with frame pointers to get a correct backtrace shown in GDB.

Also, the backtrace test for a thread currently executing a syscall now
traces the  'Genode::Thread_base::join()' function instead of
'Genode::sleep_forever()', because base-nova has a custom implementation
of 'Genode::sleep_forever()' with a different backtrace than on Fiasco.OC.

Fixes #1061.