SafeStack: Disable Darwin support

Summary:
Darwin support does not appear to be used as evidenced by the fact that
the pthread interceptors have never worked and there is no support for
other common threading mechanisms like GCD.

Reviewers: pcc, eugenis, kubamracek

Reviewed By: pcc, kubamracek

Subscribers: kubamracek, mgorny, delcypher, llvm-commits, #sanitizers, kcc

Differential Revision: https://reviews.llvm.org/D50718

llvm-svn: 339719