Use Qemu to build for non-native architecture on foreign distributions

[Andy Tai]

I am running GNU Guix on Ubuntu, on x86-64.  Sometimes I need to build
packages for 64-bit ARM (aarch64).  The GNU Guix
manual shows how to set this up on GNU Guix System (as native
distribution, using Qemu
to emulate the non-native architecture such as aarch64), but I find no
info on how to do this if I am running GNU Guix on a foreign
distribution.   Any pointer on this is appreciated.