x86: rmb() can be weakened according to new Intel spec.

x86: rmb() can be weakened according to new Intel spec.

Both Intel and AMD agree that, from a programmer's viewpoint:
 Loads cannot be reordered relative to other loads.
 Stores cannot be reordered relative to other stores.

Intel64 Architecture Memory Ordering White Paper
<http://developer.intel.com/products/processor/manuals/318147.pdf>

AMD64 Architecture Programmer's Manual, Volume 2: System Programming
<http://www.amd.com/us-en/assets/content_type/\
 white_papers_and_tech_docs/24593.pdf>

Signed-off-by: Keir Fraser <keir.fraser@eu.citrix.com>