[tsan] Use pthread_sigmask instead of sigprocmask to block signals in a thread on Darwin

On Darwin, sigprocmask changes the signal mask for the entire process. This has some unwanted consequences, because e.g. internal_start_thread wants to disable signals only in the current thread (to make the new thread inherit the signal mask), which is currently broken on Darwin. This patch switches to pthread_sigmask.

Differential Revision: https://reviews.llvm.org/D35016

llvm-svn: 307212