[tsan] Allow -fsanitize=thread for iOS-style simulator targets

Update the clang driver to allow -fsanitize=thread when targeting x86_64 iOS and tvOS
simulators. Also restrict TSan targeting OS X to only be supported on x86_64 and not i386.

Differential Revision: http://reviews.llvm.org/D18280

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263913 91177308-0d34-0410-b5e6-96231b3b80d8