Add thread suspend/resume signals public setters (POSIX threads)

Add thread suspend/resume signals public setters (POSIX threads)

* include/gc.h (GC_set_suspend_signal, GC_set_thr_restart_signal):
Add public function declaration (to specify non-default signals to
suspend/resume threads).
* include/gc.h (GC_get_suspend_signal, GC_get_thr_restart_signal):
Update comment.
* misc.c (GC_set_suspend_signal, GC_set_thr_restart_signal): Add
public no-op function (only for Darwin, OpenBSD, Win32 and NaCl).
* pthread_stop_world.c (GC_set_suspend_signal,
GC_set_thr_restart_signal): Add public setter to alter the default
signals used to suspend and resume threads (only if not OpenBSD or
NaCl); has no effect if GC is initialized.