`CMAKE_<LANG>_COMPILER` may be a list if it was defined by a toolchain
file. In this case we move the args to `CMAKE_<LANG>_COMPILER_ARG1`.
If `CMAKE_<LANG>_COMPILER` is not a list, then it might have been cached
by a previous run that split the `CC` or `CXX` environment variable into
`CMAKE_<LANG>_COMPILER` and `CMAKE_<LANG>_COMPILER_ARG1`. In this
latter case, avoid clobbering `CMAKE_<LANG>_COMPILER_ARG1`.
Fixes: #23358