Fix GC_sig_suspend initialization when non-constant SIGRTMIN used

Fix GC_sig_suspend initialization when non-constant SIGRTMIN used

* pthread_stop_world.c (SIGNAL_UNSET): New macro.
* pthread_stop_world.c (GC_sig_suspend, GC_sig_thr_restart): Initialize
to SIGNAL_UNSET (instead of SIG_SUSPEND/THR_RESTART).
* pthread_stop_world.c (GC_get_suspend_signal): Return SIG_SUSPEND if
GC_sig_suspend is SIGNAL_UNSET.
* pthread_stop_world.c (GC_get_thr_restart_signal): Return
SIG_THR_RESTART if GC_sig_thr_restart is SIGNAL_UNSET.
* pthread_stop_world.c (GC_unblock_gc_signals): Add assertion for
initialization of GC_sig_suspend and GC_sig_thr_restart.
* pthread_stop_world.c (GC_stop_init): Replace SIGNAL_UNSET value for
GC_sig_suspend, GC_sig_thr_restart to SIG_SUSPEND and SIG_THR_RESTART,
respectively.