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