Move architecture depending CMake variables into toolchain file

[domcharrier]

The variable 'CMAKE_Fortran_FLAGS' should only be set by the cmake toolchain,

• either by calling cmake -DCMAKE_Fortran_FLAGS=" -ffree-form -cpp -ffree-line-length-none -fmax-errors=5 -std=f2008 -fno-underscoring or
• set in a toolchain file and added with 'cmake -DCMAKE_TOOLCHAIN_FILE='.

[fluidnumerics-joe]

Hey @cgmb - Is this ticket still relevant ?

[cgmb]

Surprisingly, yes. At least, I think so. We're using CMake features for some of these, but I think we do want to better leverage toolchain files. I'm open to differing opinions, of course.