Clang counterpart change for fuzzer FreeBSD support

Summary: Providing fuzzer sanitizer support for FreeBSD (but only for X86/64 architectures).

Reviewers: kimgr, EricWF, martell

Reviewed By: martell

Patch by David CARLIER.

Subscribers: krytarowski, kimgr, emaste, cfe-commits

Differential Revision: https://reviews.llvm.org/D41809

llvm-svn: 322469