arc: 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: I056afb0406cabe0e1ce2612904e727ccce5f6308
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>