Enable ubsan on NetBSD

Summary:
Part of the code inspired by the original work on libsanitizer in GCC 5.4 by Christos Zoulas.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, vitalybuka, kcc, filcab, fjricci

Reviewed By: fjricci

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

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36483

llvm-svn: 310412