Fill the gaps for armv7, powerpc64 and riscv64

Fill the gaps for armv7, powerpc64 and riscv64

These still fail early on due to the opendev(3)/diskmap(4) race condition,
so only hook them up after the kernel is fixed.

Note to self: some distrib/${MACHINE}/ramdisk/install.md pass explicit
newfs(8) flags -- this might be needed here;  test once all the relevant
kernel and installboot(8) bugs are squashed and this regress suite can be
run normally.