[tsan] Replace pthread semaphore in signal_cond.cc with barrier_wait

Pthread semaphores are not available on OS X. Let's replace sem_wait/sem_post with barrier_wait, which makes the test pass on OS X. I know that sem_wait/sem_post is intercepted by TSan, whereas barrier_wait is TSan-invisible, but the purpose of the test is not affected by this.

Also, let's properly initialize the mutex and cond variables.

Differential Revision: http://reviews.llvm.org/D14924

llvm-svn: 253980