i386 and amd64 AVX support

[David Laight <david%l8s.co.uk@localhost>]

I've committed code to the amd64 and i386 kernels that enables
AVX for userspace.
In particular the high ymm registers should be saved on context switches.

Any additional testing would be welcome.
At the moment there is no support for gdb and the ymm registers are not
written to core dumps, nor available to signal handlers.

Note that the ymm registers are caller-saved so should be don't care
on all system calls, so context switches from interrupt routines are
needed to actually test whether they are saved properly.

The code should also support the upcoming AVX-512, although stealing
another 2k from the kernel stack might cause problems!

        David

-- 
David Laight: david%l8s.co.uk@localhost