ghcjsHEAD: don’t just copy & replace, reuse same code (#20493)

* ghcjsHEAD: don’t just copy & replace, reuse same code

The HEAD file was just copied over and a few hashes replaced.
Most of these hashes were already function arguments anyway, so no reason to not
abstract over that.

* ghcjs/base: factor out stage1Packages & regenerate for HEAD

The stage1Packages have changed on ghcjs head, this adds the new list.