docker: add linux-user powered cross builder for QEMU

We can't use cross compilers in the current Debian stable and Debian
sid is sketchy as hell. So for powerpc fall back to dog-fooding our
own linux-user to do the build.

As we can only build the base image with a suitably configured
source tree we fall back to checking for its existence when we can't
build it from scratch. However this does mean you don't have to keep
a static powerpc-linux-user in your active configuration just to
update the cross build image.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>