x86: Use symbolic MSR names

Use symbolic MSR names instead of hardcoding the MSR index.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
LKML-Reference: <1279371808-24804-2-git-send-email-brgerst@gmail.com>
Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>