LP#1520627 -- allow compat 32 bit architecture to be configurable at boot time

[Andy Whitcroft]

We want to be able to use the 32 bit compatibility mode on arm64 to
build armhf binaries as we do on x86 so that we can share the same pool
of builders for both Debian architectures.  This works except that the
default compatibility version armv8l is higher than that supported by
Ubuntu 32 bit userspace.  This triggers miss-compilation of packages.

This following patch makes the 32 bit compatibility version configurable at
boot time via the kernel command line.  This has been tested both locally
and on builders.  The builders are currently running wily as the latest
stable, so we need this change there.

Proposing for SRU to wily.

-apw