build: introduce an option for fuzzing-friendly mode

Patch introduce a macro FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION. It is
a common build macro recommended in libFuzzer's documentation [1].

1. https://llvm.org/docs/LibFuzzer.html#fuzzer-friendly-build-mode

Needed for #4826

NO_CHANGELOG=build
NO_DOC=build
NO_TEST=build