gcc: fix Fortran support

This commit fixes the Fortran support in the gcc package by making
sure that the runtime Fortran library is properly installed to the
target.

It also takes this opportunity to slightly reword the Config.in option
help text, to no longer mention gcc < 4.2 since they are no longer
supported in Buildroot, and to have a more meaningful explanation.

Finally, it fixes the gcc-final.mk code to use the correct
BR2_INSTALL_FORTRAN option rather than the non-existing
BR2_GCC_CROSS_FORTRAN option, to decide whether Fortran support should
be enabled or not in the cross-compiler.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>