OpenBSD UBsan support, cmake part

Summary:
On OpenBSD no multi arch support
Enabling only UBsan and disabling Asan.

Patch by: David CARLIER

Reviewers: krytarowski, vitalybuka

Reviewed By: vitalybuka

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

Differential Revision: https://reviews.llvm.org/D44017

llvm-svn: 326646