x86: make stop_this_cpu looks exactly equal in both arches

with the hlt_works change, it is possible to have i386
and x86_64 stop_this_cpu() looking exactly the same. They
can, after that, be merged.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>