swift234

[build-script] Make sure that we build using an external compiler-rt on /all/ platforms, not just OS X.

This is important for a few reasons:

1. It ensures that on all platforms we use the just built clang to compile
compiler-rt. This ensures that the clang used to compiler-rt has all of the
features necessary to compile compiler-rt in the face of changes in the
underlying implementation.

2. It ensures that all platforms build compiler-rt in the same manner.