[rt] Use sigemptyset instead of sigfillset.

When using sigfillset, all signals are masked. Unfortunately,
this includes the signals that MPS requires, which leads to
deadlocks within MPS when it tries to stop the world.

We don't need to mask any signals here, so using sigemptyset
seems safe and is what the code did here before this code was
added.
[rt] Cleanup FreeBSD exception code.

The SEGV handler wasn't used / needed and was mostly commented out.
[rt] We no longer need chain_sigaction.

MPS is now using modern signal handling facilities so we no
longer need to work around it.
Merge pull request #482 from waywardmonkeys/signal-fixes

Signal fixes