cmake-utils.eclass: Split multi-arg CC/CXX/FC correctly, #542530

cmake-utils.eclass: Split multi-arg CC/CXX/FC correctly, #542530

Split multi-argument CC/CXX/FC into a CMake array consisting of one
element containing the compiler path (name) and the other containing all
command-line options, space-separated. This is how CMake splits
the environment variables CC/CXX/FC internally.

The alternative would be to set CC/CXX/FC directly, and let CMake handle
the splitting. However, changing that had unforeseen consequences like
assembler not being set correctly (#601292) which in turn was impossible
to set via environment variables due to bugs in CMake.

Therefore, splitting the values into CMAKE_*_COMPILER correctly seems
the simplest and safest way forward, at least until we can rely on fixed
CMake versions being sufficiently deployed.