[PATCH] wire up oops_enter()/oops_exit()

Implement pause_on_oops() on x86_64.

AK: I redid the patch to do the oops_enter/exit in the existing
oops_begin()/end(). This makes it much shorter.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Andi Kleen <ak@suse.de>