restore amd64 live image support for resize root after combined mbr/gpt commit

we need to resize_gpt now, as it takes precedence over mbr/disklabel
this change brings us to behave like the evbarm images.

XXX: we don't seem to touch disklabel and MBR, but they exist. Not sure whether
that has any negative repercussions, maybe another system might regard MBR as the
sole source of truth when GPT also exists.

(maya)