[CMake] [Darwin] Bug 21562 - Add a CMake equivalent for...

[CMake] [Darwin] Bug 21562 - Add a CMake equivalent for make/platform/clang_darwin.mk in compiler_rt

Summary:
Building the builtins on Darwin platforms is a bit complicated. This is a first-pass implementation of the functionality from clang_darwin.mk into CMake.

When building the builtins on Darwin we have layers of blacklists that we apply based on platform, architecture, and minimum supported OS version.

Reviewers: bogner, filcab, bob.wilson, samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13059

llvm-svn: 248383