Handle NetBSD alias for pthread_sigmask

Summary:
Add a new helper macro TSAN_INTERCEPTOR_NETBSD_ALIAS_THR2
that handles pthread(3)/libc aliases in scenarios when a name in both
libraries differs not just in prefix namespace.

Handle TSan pthread_sigmask mangling accordingly into __libc_thr_sigsetmask.

Reviewers: vitalybuka, joerg

Reviewed By: vitalybuka

Subscribers: kubamracek, jfb, llvm-commits, #sanitizers, mgorny

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D53863

llvm-svn: 345627