cc/gcc: fortran requires GMP and MPFR

Even if gcc itself does not require GMP or MPFR (eg. gcc-4.2 and before
don't), building the fortran frontend always required those companion
libraries.

Select them if the Fortran language is selected.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

 reporter_comment="EXPERIMENTAL MIPS-64 n32 toolchain."