Add weak libfuzzer hook fallbacks and macOS-only CI job to test it #3681
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On Windows, weak symbols aren't supported like they are on Linux. So, we only add the __attribute__((weak)) if it is not on Windows.
It prevents the following error on Windows - fatal error C1189: #error: The C++ Standard Library forbids macroizing the keyword "false"
This was finally pointed out because of the CI job I set up.
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Regarding #3649, on macOS, linking libafl_libfuzzer fails when optional libfuzzer hook symbols are not provided, since the linker does not tolerate unresolved references even when the hooks are intended to be optional.
To fix this, I added weak fallback implementations for the custom mutator and crossover hooks into FuzzerInterceptors.cpp. They ensure that the symbols always exist at link time, and still allow for user-defined hooks to override them if they are present.
I also added a macOS-only CI job to build libafl_targets and libafl_libfuzzer to check if it works.
Let me know if there's anything else I should change!