[CMake] merge add_compiler_rt_runtime and add_compiler_rt_darwin_runtime into a single function

Summary: This refactoring moves much of the Apple-specific behavior into a function in AddCompilerRT. The next cleanup patch will remove more of the if(APPLE) checks in the outlying CMakeLists.

This patch adds a bunch of new functionality to add_compiler_rt_runtime so that the target names don't need to be reconstructed outside the call. It also updates some of the call sites to exercise the new functionality, but does not update all uses fully. Subsequent patches will further update call sites and move to using the new features.

Reviewers: filcab, bogner, kubabrecka, zaks.anna, glider, samsonov

Subscribers: beanz, rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D12292

llvm-svn: 245970