cmake: introduce GCC_COMPILER_VERSION variable

Introduce GCC_COMPILER_VERSION to be used in Zephyr CMake when knowledge
of the current GCC Compiler version is needed and
CMAKE_C_COMPILER_VERSION has not been defined yet.

CMAKE_C_COMPILER_VERSION is not defined until project() is called.

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