[compiler-rt][CMake] Pass down LLVM_LIT_ARGS in runtime build

We should also pass down the LLVM_LIT_ARGS in runtime build mode,
so that the runtime tests can be well controlled as well.

We actually passed this down in clang/runtime/CMakeLists.txt
But not for calls from llvm/runtime/CMakeLists.txt.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D83565