Split many_tls_keys.cpp into two tests

Summary:
many_tls_keys_pthread.cpp for TSD
many_tls_keys_thread.cpp for TLS

The TSD test is unsupported on NetBSD as it assumes TLS used internally.
TSD on NetBSD does not use TLS.

Reviewers: joerg, vitalybuka, mgorny, dvyukov, kcc

Reviewed By: vitalybuka

Subscribers: jfb, llvm-commits, #sanitizers

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D67428

llvm-svn: 371757