core: unblock fatal (crashing) signals in all threads

Currently all non-main threads have all the signals blocked, however
according to `man pthread_sigmask':

> If SIGBUS, SIGFPE, SIGILL, or SIGSEGV are generated while they
> are blocked, the result is undefined, unless the signal was
> generated by kill(2), sigqueue(3), or raise(3).

On macOS they are actually blocked, causing the faulting instruction
to loop indefinitely. While on Linux they are not blocked, however the
signal handler registered by sigaction is not executed. Don't block them.

Closes #8023
Closes #8083

NO_DOC=bugfix

(cherry picked from commit 706bfea4)