package/gcc: wrap gfortran

gfortran supports all options supported by gcc, so it can and should be called
via the toolchain wrapper.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>