tools: toolchain: prepare: aloow running on non-x86
tools: toolchain: prepare: aloow running on non-x86

`prepare` builds a multiarch image using qemu emulation. It turns
out that aarch64 emulation is slowest (due to emulating pointer
authentication) so it makes sense to run it on an aarch64 host. To do
that, we need only to adjust the check for qemu installation.

Unfortunately, docker arch names and Linux arch names are different,
so we have to add an ungainly translation, but otherwise it is a
simple loop.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla