We wish to use the arm64 buildds to build armhf binaries in 32bit chroots.
To make this work we need uname to return armv7l machine type.  To achieve
this add a kernel command line override for the 32bit machine type.
Add compat_uts_machine=<type> to allow the LINUX32 personality to return
that type for uname.

Signed-off-by: Andy Whitcroft <apw@canonical.com>