x86, 386 removal: Document Nx586 as a 386 and thus unsupported

Per Alan Cox, Nx586 did not support WP in supervisor mode, making it a
386 by Linux kernel standards.  As such, it is too unsupported now.

Reported-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Link: http://lkml.kernel.org/r/20121128205203.05868eab@pyramind.ukuu.org.uk
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>