[RFC] arm: Add for atomic half word exchange

On Tue, May 19, 2015 at 11:20:11AM +0000, Sarbojit Ganguly wrote:

Yes, I will try to do that. OTOH, I saw that  there was a discussion on
removal of bad_xchg() altogether. Perhaps that approach be better than
adding this half word exchange?

The only possibility for removal of __bad_xchg() is to remove it's
_definition_ only, not its callsite, so that we get a _link_ time
error for use cases we don't support.

Removing its callsite leaves us open to code malfunction: xchg()
effectively becomes a no-op for sizes which are not supported, and
that's a _very_ bad thing to happen.

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