cmake: Deprecate 'xtools' toolchain variant

The `xtools` toolchain variant (aka. Crosstool-NG) was originally
introduced to be used with the Crosstool-NG-based Zephyr SDK
toolchains (i.e. sdk-ng).

This is no longer necessary because the current Zephyr SDK (sdk-ng)
already has its own `zephyr` toolchain variant, which fully replaces
the `xtools` toolchain variant, and the `xtools` toolchain variant
serves no purpose at all.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>