[CMake] Add OBJECT_LIBS option to add_compiler_rt_runtime, and refactored asan call site to use it.

Summary: This is one more step to allow us to eliminate platform-specific code from the library CMakeLists files. Subsequent patches will refactor all call sites to use this and other new features.

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

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12339

llvm-svn: 246047