Add initial XRay support for NetBSD

Summary:
Reuse the existing FreeBSD code as it is.

Sponsored by <The NetBSD Foundation>

Reviewers: dberris, rnk, vitalybuka

Reviewed By: dberris

Subscribers: mclow.lists, emaste, mgorny, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D43370

llvm-svn: 325345