[compiler-rt] Intercept the bcmp() function.

Summary:
I have not introduced a separate hook for `bcmp()` as I don't think there
should be any reason for a sanitizer to treat it differently from `memcmp()`.

This is only enabled when building on POSIX with GNU extensions.

Context: this is to avoid losing coverage when emitting `bcmp() == 0` instead
of `memcmp() == 0` in llvm, see https://reviews.llvm.org/D56593.

Reviewers: mgorny, krytarowski, vitalybuka, dvyukov

Subscribers: kubamracek, dberris, delcypher, jdoerfert, #sanitizers, llvm-commits, jyknight

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D58379

llvm-svn: 354851