[CMake] Cleanup add_compiler_rt_object_library to be platform-agnostic

Summary:
This change takes darwin-specific goop that was scattered around CMakeLists files and spread between add_compiler_rt_object_library and add_compiler_rt_darwin_object_library and moves it all under add_compiler_rt_object_library.

The goal of this is to try to push platform handling as low in the utility functions as possible.

Reviewers: rnk, samsonov

Reviewed By: rnk, samsonov

Subscribers: rnk, rsmith, llvm-commits

Differential Revision: http://reviews.llvm.org/D10250

llvm-svn: 239498