Revert "x86: Side-step lguest problem by only building cmpxchg8b_emu for pre-Pentium"

Revert "x86: Side-step lguest problem by only building cmpxchg8b_emu for pre-Pentium"

This reverts commit 75eda5bcd238b96b7c63200bbfa7cbe06f84d2f6.

As Linus said in 24e1723366a: "There was something really messy about
cmpxchg8b and clone CPU's, so if you enable it on other CPUs later, do it
carefully."

This breaks lguest for those configs, but we can fix that by emulating
if we have to.

Fixes: http://bugzilla.kernel.org/show_bug.cgi?id=14884
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>