pmb/aportgen/gcc.py: add LANG_D=false
Alpine's APKBUILD for gcc supports a new language that we don't need, so disable it just like the irrelevant ones.
Related: pmaports!839 (merged)
This makes "pmbootstrap aportgen gcc-aarch64" etc. generate proper pmaports again, which is currently broken. So this is an urgent fix, merging as soon as CI passes.