[sanitizer] Add cloak_sanitizer_signal_handlers runtime option (#162746)
If set, signal/sigaction will pretend that the sanitizers did not preinstall any signal handlers. If a user successfully installs a signal handler, it will not be cloaked.
The flag is currently off by default, which means this patch should not affect the behavior of any sanitizers.
This can be useful in an ecosystem where:
1) there exists a library that will install a signal handler iff it does not detect a preinstalled signal handler (a heuristic to prevent overriding user-installed exception handlers etc.)
2) the aforementioned library is linked in to some, but not all, apps
3) user-installed signal handlers are intended to have the highest priority, followed by the library-installed signal handler, and then the sanitizer's signal handler
The flag is in sanitizer_common, though it is currently only supported in ASan, LSan, MSan, TSan and UBSan.
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