[jit] Unblock more critical signals at startup.

[jit] Unblock more critical signals at startup.

Unblock the GC suspend/restart signals and SIGCHLD. Those are required to make the GC
and process watching work.

The SIGCHLD issue was found on mod_mono that fork a process with that signal blocked.

The GC signals are been preemptively unblocked as we can never know what sort of sig mask
we'll start with.