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

src/shm_linux.h

@@ -106,7 +106,16 @@ class CleanupHooks {
         // Search threads may still be running, so skip munmap (but still perform
         // other cleanup actions). The memory mappings will be released on exit.
         SharedMemoryRegistry::cleanup_all(true);
-        _Exit(128 + sig);
+
+        // Invoke the default handler, which will exit
+        struct sigaction sa;
+        sa.sa_handler = SIG_DFL;
+        sigemptyset(&sa.sa_mask);
+        sa.sa_flags = 0;
+        if (sigaction(sig, &sa, nullptr) == -1)
+            _Exit(128 + sig);
+
+        raise(sig);
     }
 
     static void register_signal_handlers() noexcept {