libFuzzer, OpenBSD support

Summary:
- Enabling libfuzzer on OpenBSD
- OpenBSD can t support asan, msan ... the tests can t be run.

Patch by David CARLIER

Reviewers: eugenis, phosek, vitalybuka

Reviewed By: vitalybuka

Subscribers: srhines, mgorny, krytarowski, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D44877

llvm-svn: 329631