Following up the discussion on D77638 (and following rGd6cfed6060c283dc4a6bf9ca294dcd732e8b9f72 as example), defining `__sanitizer_cov_bool_flag_init` as the weak interface functions in various compiler-rt/ files.
Not included here:
* compiler-rt/lib/fuzzer/FuzzerTracePC.cpp [need to implement the handler function]
* clang/lib/Driver/SanitizerArgs.cpp [tied to FuzzerTracePC.cpp]
* compiler-rt/lib/sanitizer_common/sanitizer_coverage_win_sections.cpp [this file is about windows specific tricks]
I'll put all the fuzzer related changes together in another diff.