gcc: Turn FORTRAN into a weak assignment

gcc: Turn FORTRAN into a weak assignment

This ensures that it can be turned on from a distro layer
config metadata e.g. local.conf without forcing overrides

currently we have do something like

FORTRAN:forcevariable = ",fortran"
RUNTIMETARGET:append:pn-gcc-runtime = " libquadmath "
TOOLCHAIN_TARGET_TASK:append = " gfortran"

to enabled fortran support from distro conf file.

After this change we can do it with

FORTRAN = ",fortran"
RUNTIMETARGET:append:pn-gcc-runtime = " libquadmath "
TOOLCHAIN_TARGET_TASK:append = " gfortran"

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>