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.