powerpc: Save the TAR register earlier

This moves us to save the Target Address Register (TAR) a earlier in
__switch_to.  It introduces a new function save_tar() to do this.

We need to save the TAR earlier as we will overwrite it in the transactional
memory reclaim/recheckpoint path.  We are going to do this in a subsequent
patch which will fix saving the TAR register when it's modified inside a
transaction.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Cc: <stable@vger.kernel.org> [v3.10]
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>