gcc-cross: Fix linker and fortran symlinks

gcc-cross: Fix linker and fortran symlinks

If we used -fuse-ld gcc option, then it does not work
ending in

collect2: fatal error: cannot find 'ld'                                                                                                                                                                                                       compilation terminated.

This is because we are not creating proper symlinks for BFD
and Gold linker in gcc installation

Secondly, we end up with dangling fortran compiler symlinks
if fortran is not enabled when confguring gcc, therefore
create these symlinks only when fortran support is enabled
in gcc

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>