[CMake][compiler-rt] Support clang-cl in CompilerRTMockLLVMCMakeConfig

clang-cl doesn't support -dumpmachine directly, so we need to
preface it with /clang: in order to get this probing function
to work.

This is needed in order to run cmake directly on the runtimes
directory.

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D143557