cmake: manual handling of CMakeCheckCompilerFlag.cmake inclusion

Fixes: #30232

This is a workaround for #30232.

During Zephyr CMake invocation a plain C compiler is used for DTS.
This results in the internal `CheckCompilerFlag.cmake` being included
by CMake

Later, when the full toolchain is configured, then
`CMakeCheckCompilerFlag.cmake` is included.
This overloads the `cmake_check_compiler_flag()` function, thus
causing #30232.

By manualy loading `CMakeCheckCompilerFlag.cmake` then
`CheckCompilerFlag.cmake` will overload the functions (and thus win the
battle), and because `include_guard(GLOBAL)` is used in
`CMakeCheckCompilerFlag.cmake` this file will not be re-included later.

It also prints a warning informing the user of the issue.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>