[RFC] arm: Add for atomic half word exchange

[Peter Zijlstra]

On Wed, May 20, 2015 at 05:09:34AM +0000, Sarbojit Ganguly wrote:
> Yes, the main advantage of Qspinlock code can be observed in NUMA but
> when I tested in an embedded system, a slight advantage was observed.

OK, great!

This is the first !x86 port I'm aware of and a load-store arch at that,
so its good to hear our efforts at making it generic actually paid off.