Fix deadlock (and double lock) in explicit thread suspend/resume

Fix deadlock (and double lock) in explicit thread suspend/resume

* pthread_stop_world.c (GC_suspend_handler_inner)
[GC_ENABLE_SUSPEND_THREAD]: If SUSPENDED_EXT flag then set
stop_info.stack_ptr, call sem_post(suspend_ack_sem), and call
suspend_self_inner instead of GC_do_blocking(suspend_self_inner).
* pthread_stop_world.c [GC_ENABLE_SUSPEND_THREAD] (GC_suspend_thread):
No-op if already suspended; UNLOCK before GC_do_blocking (if
self-suspend); add TODO about GC_retry_signals; clear SUSPENDED_EXT
flag if RAISE_SIGNAL failed with ESRCH code; sem_wait(suspend_ack_sem)
to let the suspend handler to lookup the thread and store stack_ptr
(and save registers if needed).
* pthread_stop_world.c (GC_suspend_all, GC_start_world): Skip threads
with SUSPENDED_EXT flag.