cmake: propagate endianess to CMake CMAKE_<lang>_BYTE_ORDER setting

Fixes: #45270

Zephyr Kconfig defines the settings BIG_ENDIAN.
Propagate this setting to the corresponding CMAKE_C_BYTE_ORDER and
CMAKE_CXX_BYTE_ORDER variables.

This also ensures that the CMake function 'is_big_endian()' reports the
correct endianess.

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