Handle NetBSD specific indirection of libpthread functions

Summary:
Correct handling of libpthread(3) functions in TSan/NetBSD:

 - pthread_cond_init(3),
 - pthread_cond_signal(3),
 - pthread_cond_broadcast(3),
 - pthread_cond_wait(3),
 - pthread_cond_destroy(3),
 - pthread_mutex_init(3),
 - pthread_mutex_destroy(3),
 - pthread_mutex_trylock(3),
 - pthread_rwlock_init(3),
 - pthread_rwlock_destroy(3),
 - pthread_rwlock_rdlock(3),
 - pthread_rwlock_tryrdlock(3),
 - pthread_rwlock_wrlock(3),
 - pthread_rwlock_trywrlock(3),
 - pthread_rwlock_unlock(3),
 - pthread_once(3).

Code out of the libpthread(3) context uses the libc symbols
that are prefixed with __libc_, for example: __libc_cond_init.

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: kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40243

llvm-svn: 318673