Fix build for BridgeSupport in Mac OS X 10.6

This was accomplished by changing the compiler-rt code so that it avoids building for armv6 architectures whenever the target OS is Darwin.

Unfortunately, Clang is packed up in a zip file, so you can't see the diff without pulling and comparing yourself.

This does not fix the build for OS X 10.8. In 10.8, the process that creates all the BridgeSupport files for the current OS while running make is extremely unstable. You'll see it crash at least once. It will never complete successfully.

I do not know how it performs in 10.7, yet. If you'd like, I can set up a virtual machine and see.