Issue #11753: faulthandler thread uses pthread_sigmask()

Issue #11753: faulthandler thread uses pthread_sigmask()

The thread must not receive any signal. If the thread receives a signal,
sem_timedwait() is interrupted and returns EINTR, but in this case,
PyThread_acquire_lock_timed() retries sem_timedwait() and the main thread is
not aware of the signal. The problem is that some tests expect that the main
thread receives the signal, not faulthandler handler, which should be
invisible.

On Linux, the signal looks to be received by the main thread, whereas on
FreeBSD, it can be any thread.