OpenBSD UBsan support enabling SANITIZER_OPENBSD widely

Summary: Adding OpenBSD platform

Patch by: David CARLIER

Reviewers: krytarowski, vitalybuka

Reviewed By: vitalybuka

Subscribers: srhines, kubamracek, fedor.sergeev, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D44049

llvm-svn: 326651