Support pthread_key_create symbol alias in MSan/NetBSD

Summary:
NetBSD uses indirection symbol for a set of threading functions.

Add alias to handle __libc_thr_keycreate the same way as pthread_key_create.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, dvyukov, eugenis, vitalybuka

Reviewed By: vitalybuka

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40767

llvm-svn: 319868