Fix GC_dirty_init by avoiding GC_get_suspend_signal if no threads support

Fix GC_dirty_init by avoiding GC_get_suspend_signal if no threads support
(fix commit '05daaee')

* os_dep.c (GC_dirty_init): Pass SIG_SUSPEND instead of
GC_get_suspend_signal() to sigaddset() if no THREADS (or if
GC_OPENBSD_THREADS, or GC_WIN32_THREADS or NACL).
* pthread_support.c (pthread_sigmask): Assert GC_get_suspend_signal()
does not return -1 (only if not GC_NO_PTHREAD_SIGMASK).