[tsan] Skip building libcxx_tsan on OS X

The TSan-instrumented version of libcxx doesn't even build on OS X at this point. Let's skip it from the OS X build for now, since most of TSan functionality doesn't depend on it. This will enable `check-tsan` to be run.

Differential Revision: http://reviews.llvm.org/D14486

llvm-svn: 252455