Handle NetBSD specific indirection of libpthread functions

Summary:
Correct handling of three libpthread(3) functions on NetBSD:

 - pthread_mutex_lock(3),
 - pthread_mutex_unlock(3),
 - pthread_setcancelstate(3).

Code out of the libpthread(3) context uses the libc symbols:

 - __libc_mutex_lock,
 - __libc_mutex_unlock,
 - __libc_thr_setcancelstate.

The threading library (libpthread(3)) defines strong aliases:

 - __strong_alias(__libc_mutex_lock,pthread_mutex_lock)
 - __strong_alias(__libc_mutex_unlock,pthread_mutex_unlock)
 - __strong_alias(__libc_thr_setcancelstate,pthread_setcancelstate)

This caused that these functions were invisible to sanitizers on NetBSD.
Intercept the libc-specific ones and add them as NetBSD-specific aliases
for the common pthread(3) ones.

NetBSD needs to intercept both functions, as the regularly named ones
are used internally in libpthread(3).

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, dvyukov, vitalybuka

Reviewed By: dvyukov

Subscribers: llvm-commits, kubamracek, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40241

llvm-svn: 318646