The commit "Split GNU compiler information files" intended to move GNU
flags from the platform-wide Platform/SunOS.cmake module into
Platform/SunOS-GNU-<lang>.cmake
using a helper module Platform/SunOS-GNU.cmake to consolidate flags.
However, it accidentally put Fortran flags in the C language module and
left out the Fortran module altogether. This fixes those mistakes.
This moves GNU compiler flags into new-style modules
Compiler/GNU-<lang>.cmake
Platform/<os>-GNU-<lang>.cmake
We use language-independent helper modules
Compiler/GNU.cmake
Platform/<os>-GNU.cmake
to define macros consolidating the information.