Retiring two more: Atmel and Xscale

[Warner Losh]

I'd like to retire Atmel and Xscale support prior to 12. The atmel stuff
has been dodgy for a long time and generally lacks the memory to run
FreeBSD well. We have no current users of it.

The Xscale stuff is old enough to not be relevant. There's no known users.
Part of it is already set to go with the big endian stuff, but is there a
reason to keep the rest?

Warner