cmake: Improve acceptance of arbitrary arguments in -P script mode

The fix in commit e4f1b301fe (cmake: Allow arbitrary args passed to
CMake script, 2020-05-04, v3.18.0-rc1~211^2) only applied to "cache"
arguments like `-DFOO`.  Extend the fix to allow arbitrary arguments
that collide with other CMake arguments like `-S` and `-B`.