[CMake] Refactoring add_compiler_rt functions for darwin runtimes.

Summary: This patch consolidates add_compiler_rt_osx_static_runtime and add_compiler_rt_darwin_dynamic_runtime into a single new function add_compiler_rt_darwin_runtime.

Reviewers: filcab, samsonov, bogner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12106

llvm-svn: 245317