cmake: move deprecated code to FindDeprecated.cmake

Move deprecated XCC_USE_CLANG setting to deprecated module.

Deprecated and move the use of CROSS_COMPILE to deprecated module as we
have had ZEPHYR_TOOLCHAIN_VARIANT==cross-compile for a long time.

Deprecated code is now loaded with a single line:
> find_package(Deprecated COMPONENTS XCC_USE_CLANG CROSS_COMPILE)

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