xvoidstrap: allow bootstrapping cross archs

[CameronNemo]

This adds the necessary logic to support bootstrapping architectures
that are different from the host architecture.

/proc needs to be mounted because a base-files invocation of install
uses it to chmod tmp and var/tmp...

[leahneukirchen]

Where is the part that uses qemu-static? oO

[CameronNemo]

It is assumed that binfmt is installed (perhaps a bad assumption, should maybe add a test in there), so pretty much everything in the chroot is running with qemu-user-static if necessary.

[leahneukirchen]

Ah yes. I think you can set up a binfmt_misc with flag F and it will work inside the chroot without issues, perhaps that is an easier way?

[CameronNemo]

It is assumed that binfmt is installed (perhaps a bad assumption, should maybe add a test in there)

Added a little test.

I think you can set up a binfmt_misc with flag F and it will work inside the chroot without issues, perhaps that is an easier way?

It would easier for this script if that was already set up, however setting it up requires knowing the magic values for the relevant qemu architecture. mklive hardcodes them (https://github.com/void-linux/void-mklive/blob/master/lib.sh.in#L168), but another option is to parse the binfmt files that are installed into /var/lib/binfmts. Both options are more involved than just copying the relevant binary in.