Make it an error to build an ARM kernel with COMPAT_FREEBSDn where n < 10.
We changed the ABI for ARM in 10, an removed support for the old ABI in 11,
as such binaries from these releases are unable to be run on a head kernel.

Reviewed by: bz, emast
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D5652