Permit -fxray-instrument for NetBSD/amd64

Summary:
Use the same branch as FreeBSD and OpenBSD.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, dberris, vitalybuka

Reviewed By: vitalybuka

Subscribers: emaste, llvm-commits

Differential Revision: https://reviews.llvm.org/D46737

llvm-svn: 332070