build: don't attempt to run config.guess on Windows

When cross-compiling LLVM to android from Windows (for LLVMSupport), we would
attempt to execute config.guess to determine the host triple since
CMAKE_SYSTEM_NAME is not Windows and CMAKE_C_COMPILER will be set to GNU or
Clang. This will fail as config.guess is a shell script which cannot be
executed on Windows. Simply log a warning instead. The user can specify the
value for this instead in those cases.