python.mk: Improve CMake/Python integration

Suport FindPython.cmake, FindPython3.cmake, FindPython2.cmake modules by
adding Python{,2,3}_EXECUTABLE to CMAKE_ARGS in python.mk.

CMake supports more than one way to search for python. Currently
python.mk passes -DPython_ADDITIONAL_VERSIONS=${PYTHON_VER} to help
FindPython{Interp,Libs}.cmake modules "find" the version of python that
a port build wants to use.

The FindPython{,2,3}.cmake modules don't know anything about
Python_ADDITIONAL_VERSIONS but use Python{,2,3}_EXECUTABLE as the hint.

PR: 262109