Support rthreads introduced in OpenBSD 5.2+

Support rthreads introduced in OpenBSD 5.2+

* include/gc_config_macros.h (GC_OPENBSD_UTHREADS): New macro (defined
only if OpenBSD prior to 5.2 release and threads).
* include/private/gc_priv.h (SIG_SUSPEND): Define for OpenBSD (unless
GC_OPENBSD_UTHREADS).
* include/private/pthread_stop_world.h (thread_stop_info): Test
GC_OPENBSD_UTHREADS macro instead of GC_OPENBSD_THREADS.
* misc.c (GC_set_suspend_signal, GC_set_thr_restart_signal,
GC_get_suspend_signal, GC_get_thr_restart_signal): Likewise.
* os_dep.c (GC_dirty_init): Likewise.
* pthread_stop_world.c: Likewise.
* tests/initsecondarythread.c (main): Likewise.