use default signal handler after cleanup
With the current setup, on Linux, SIGILL (and SIGSEGV/SIGBUS etc., in the case
of a program bug) lead to no feedback if they occur after the signal handlers
are installed, they just exit silently. By invoking the default signal handler,
we can still get the appropriate feedback. This is particularly important for
feedback if someone downloads the wrong SF binary and runs it on Linux.
closes https://github.com/official-stockfish/Stockfish/pull/6554
No functional change