x86: fix irq_lock/unlock ordering bug

Memory accesses could be reordered before an irq_lock() or after an
irq_unlock() without the memory barriers.

See commit 15bc5377 for the ARM fix for
a complete description of the issue and fix.

Change-Id: Ic92a6b33f62a938d2252d68eccc55a5fb07c9114
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>
(cherry picked from commit 1f8125a4)