[tsan] On OS X, build Go runtime with -mmacosx-version-min

We're not building the Go runtime with -mmacosx-version-min, which means it'll have a minimum deployment target set to the system you're building on. Let's make the code compile (and link) with -mmacosx-version-min=10.7.

Differential Revision: http://reviews.llvm.org/D20670

llvm-svn: 271833