x86/nmi: Clean up register_nmi_handler() usage

Implement a cleaner and easier to maintain version for the section
warning fixes implemented in commit eeaaa96a3a21
("x86/nmi: Fix section mismatch warnings on 32-bit").

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Cc: Jan Beulich <JBeulich@suse.com>
Link: http://lkml.kernel.org/r/1340049393-17771-1-git-send-email-dzickus@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>