Handle pthread restart signals loss if retry_signals

Handle pthread restart signals loss if retry_signals

Issue #181 (bdwgc).

* doc/README.environment (GC_RETRY_SIGNALS, GC_NO_RETRY_SIGNALS):
Update documentation (support of restart signals loss, try signals
if compiled with ASan/MSan/TSan).
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL]
(GC_retry_signals): Set true also if ADDRESS_SANITIZER or
MEMORY_SANITIZER.
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL
&& !GC_NETBSD_THREADS_WORKAROUND] (GC_suspend_handler_inner): Call
sem_post(GC_suspend_ack_sem) at the function end if GC_retry_signals;
update comment about the RESTART signal loss.
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL
&& !GC_OPENBSD_UTHREADS] (GC_start_world): Call
resend_lost_signals(GC_restart_all) and update n_live_threads value.
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL
&& !GC_OPENBSD_UTHREADS && !GC_NETBSD_THREADS_WORKAROUND]
(GC_start_world): Call suspend_restart_barrier() if GC_retry_signals.
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL] (GC_stop_init):
Update the message logged if GC_retry_signals.