[CMake] Fix building builtins on Linux and Darwin.

Summary:
I broke building the builtins with r245967. This fixes them on Linux and builds them properly for Darwin.

The old code could not be made to work on Darwin as a result of the refactoring of add_compiler_rt_runtime, so I had to rework the way they are built for Darwin. This solution is not ideal and will be fixed in subsequent commits. I just want to get this in so everything is working again.

Reviewers: samsonov, chh, compnerd, bogner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12500

llvm-svn: 246487