[CMake] Support undefined LLVM_NATIVE_ARCH in llvm_map_components_to_libnames().

https://reviews.llvm.org/D142405 made this function relying on the
LLVM_NATIVE_ARCH be defined, which is not necessarily the case for
third-party projects that include LLVM as their part.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D142610