Support XRay in the NetBSD driver

Summary:
While there, perform a small cleanup, reducing delta
with drivers for other OSes.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, vitalybuka, dberris

Reviewed By: dberris

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D46721

llvm-svn: 332071