[BUG] SMP signal dispatch seems racy

### Description / Steps to reproduce the issue

the signal sending logic (kill) basically does:
1. enter_critical_section
2. decide the target thread
3. check signal mask of the thread
4. make a cross call to the target cpu (nxsched_smp_call_single)

as nxsched_smp_call_single drops the critical section, there seems to be nothing to prevent the signal mask from changing in the meantime. it can end up with a signal delivered while being blocked.

similarly, the need_restore logic in sig_handler seems dangerous as it can revert unrelated changes to those fields.

### On which OS does this issue occur?

[OS: Mac]

### What is the version of your OS?

macOS 14.7

### NuttX Version

master

### Issue Architecture

[Arch: all]

### Issue Area

[Area: Kernel]

### Verification

- [X] I have verified before submitting the report.