[SanitizerCoverage] Add `__sanitizer_cov_bool_flag_init` as the weak interface functions.

Summary:
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.

Reviewers: vitalybuka

Reviewed By: vitalybuka

Subscribers: dberris, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D77857