Bear's Restaurant has race condition with deferred signal handling

[Bear's Restaurant](https://store.steampowered.com/app/1687550/Bears_Restaurant/) has a race condition with FEX's deferred signal handling. Its [game engine](https://ebitengine.org/) is based on golang, which sends SIGURG /very/ quickly for doing preemption to ensure thread fairness.

A couple of things to fix in FEX's deferred signal handling.
- If its not a realtime signal, then these shouldn't be queued and instead just set in a mask of available signals.
- Linux doesn't queue non-RT signals, expecting many non-RT signals to be lost rather than queued
- FEX should block the `sa_mask` in the signal handler of the signal getting deferred, to ensure it doesn't receive anymore of that signal.
- This would ensure FEX doesn't overrun its small signal queue, which can typically result in crashes.
- Make sure to restore signal mask once actually handling the signal

I don't have time to implement these fixes right now but I needed a tracking issue to remember it.

I originally wanted to test this engine to see it break under Asahi Linux, but apparently it uses a new enough version of golang that it no longer hits the VDSO bug, might need to try an older game using this engine to see the Asahi breakage.