WIP: Guest Signals: Correctly block raised signal, implement SA_NODEFER, sigreturn sa_mask restore

[skmp]

Overview

This is the first set of fixes from #1682

• The raised signal in the guest handler would not be blocked. As the host handler does sigreturn, we can't rely on the host handling there.
• Also implemented guest SA_NODEFER there.
• On guest signal return, the guest provided uc_sigmask is restored, not the one saved on signal entry, as the guest can modify this.

TODO

• [ ] Cleanup the code
• [ ] move more of the signal logic away from the jit dispatcher
• [ ] Make sure we don't set SA_NODEFER for the host signals, as we don't want that there
• [ ] Integrate tests from https://github.com/FEX-Emu/fex-assorted-tests-bins/tree/main/src/tests

Test todo

• [ ] Add test for procmask inside signal handler, with and without SA_NODEFER
• [ ] Add test for procmask after signal handler, with and without SA_NODEFER
• [ ] SA_NODEFER tests validating around host signal handling
• [ ] Verify guest_uctx sigmask is set correctly before entry to signal handler

[skmp]

Interestingly enough, we did have a (partially working) implementation of half the fixes there, that somehow failed to work during my tests, but works now? Peculiar.

The signal mask setting still needs to be fixed though.

[skmp]

(Closing this as there is an ongoing powergrab by @Sonicadvance1, I will migrate my work to https://github.com/skmp/fex-emu-ng.git)