32-bit Arm emulation spins at 100% during emacs build

This test case exposes a QEMU bug when crossbuilding to arm32. The bug is only
exposed on amd64 architecture or aarch64 hosts that can *only* execute
64 bit applications.

Usage:

./setup.sh # installs docker and tweaks sysctl
./qemu.sh # register qemu so you are able to run containers from other
architectures
./build.sh # try to build the docker image. Process should get stuck
in a 100% CPU loop after a while

Originally reported by, and test case developed by,
Philippe Vaucher.