x86: remove NexGen support

It is claimed that NexGen CPUs were never shipped:

   http://lkml.org/lkml/2008/4/20/179

Also, the kernel support for these chips has been broken for
a long time, the code intended to support NexGen thereby being
essentially dead.

As an outcome of the discussion that can be found using the URL
above, this patch removes the NexGen support altogether.

The changes in this patch survived a defconfig build for i386, a
couple of successful randconfig builds, as well as a runtime test,
which consisted in booting a 32-bit x86 box up to the shell prompt.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>