[sanitizer_common] Suppress warning of cast from SignalHandlerType to sa_sigaction_t (#86046)

Some buildbots (e.g.,
https://lab.llvm.org/buildbot/#/builders/18/builds/16061/steps/10/logs/stdio)
have recently started complaining about
```
cast from 'SignalHandlerType' (aka 'void (*)(int, void *, void *)') to 'sa_sigaction_t' (aka 'void (*)(int, siginfo_t *, void *)') converts to incompatible function type [-Werror,-Wcast-function-type-strict]
  219 |   sigact.sa_sigaction = (sa_sigaction_t)handler;
  ```
This patch does an intermediate cast to `'(void (*) (void))'` to suppress the warning.

N.B. SignalHandlerType has `'void*'` instead of `'siginfo_t*'` because it is typedef'ed in sanitizer_common/sanitizer_common.h, which does not have access to the header (signal.h) that defines siginfo_t; we therefore cannot fix SignalHandlerType.