Unix/Process: Don't use pthread_sigmask if we aren't built with threads

We won't link in pthreads if we weren't built with LLVM_ENABLE_THREADS
which means we won't get access to pthread_sigmask.  Use sigprocmask
instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219288 91177308-0d34-0410-b5e6-96231b3b80d8