Implement GetTls() for NetBSD

Summary:
Reuse the existing code for FreeBSD that is compatible with NetBSD.

Add NetBSD support in tests: tls_race.cc and tls_race2.cc.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, dvyukov, vitalybuka, kcc, eugenis

Reviewed By: dvyukov

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

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40105

llvm-svn: 318431