intercept libc's regexec, at least to avoid msan false positives

See e.g. https://github.com/file/file/commit/393dafa41b26a7d8ed593912e0ec1f1e7bd4e406
(valid code require a workaround to use msan)

Start with the referenced file/file commit 393dafa41b26a7d8ed593912e0ec1f1e7bd4e406 and inspect how libc's regexec interacts with MemorySanitizer. Then locate the sanitizer interceptor code and determine the scope needed to prevent the reported false positives; done means valid code no longer requires the referenced workaround.

Written by the indexing model from the issue text.