OpenBSD UBsan support common functions

Summary: Ripped off OpenBSD specific from the common Linux implementation

Patch by David Carlier

Reviewers: krytarowski, vitalybuka

Reviewed By: vitalybuka

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

Differential Revision: https://reviews.llvm.org/D44036

llvm-svn: 327674