[boehm] Implement mono_gc_get_restart_signal.
[sgen] Implement mono_gc_get_restart_signal.
[gc] Add mono_gc_get_restart_signal.
[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.
[runtime] Fix a pair of thread abort races in classic abort code.

The two races in question are:

In signal_thread_state_change we need to block the target from
further waiting before we signal it or otherwise it might miss the
broadcast asking it to wakeup.

In mono_thread_request_interruption we need to ensure we self interrupt
before returning from the signal handler.