Add dfsan_weak_hook_memcmp

Summary:
Add a weak hook to be called from dfsan's custom memcmp.
The primary user will be lib/Fuzzer.
If this works well we'll add more hooks (strcmp, etc).

Test Plan: Will be covered by lib/Fuzzer tests.

Reviewers: pcc

Reviewed By: pcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9541

llvm-svn: 236679