Implement thread suspend/resume API (Linux threads only)

Implement thread suspend/resume API (Linux threads only)
(Cherry-picked commit 42fe54a from 'gcc_boehmgc' branch.)

* pthread_stop_world.c (GC_suspend_handler): Redirect to suspension
routine if signal is received and thread is flagged SUSPENDED_EXT.
* pthread_stop_world.c (GC_brief_async_signal_safe_sleep,
suspend_self_inner, suspend_self, GC_suspend_thread,
GC_resume_thread): New function.
* include/gc.h (GC_suspend_thread, GC_resume_thread): Declare.
* include/private/pthread_support.h (SUSPENDED_EXT): Update comment.

Conflicts:
* ChangeLog
* include/gc.h
* include/private/pthread_support.h
* pthread_stop_world.c