cmake: move CMAKE_<lang>_COMPILER_FORCED into toolchain code.

To prepare for more modular approach move the compiler forced out of
boilerplate and into the specific toolchain cmake code (generic/target).

Code is added to both generic and target toolchain modules to allow
future use of one module without requiring a load of the other module.

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