This patch changes 32 bit loads and stores (which did trigger
undefined behaviour when the pointer wasn't aligned) to use the
xx_storel_32 synonym. This should also just generate a MOVD and is
less verbose to boot!

The patch also changes store_buffer_horz_8 to take its SSE register by
value rather than by pointer. The most restrictive ABI for passing SSE
registers by value is win32, where you can pass at most 3. There's
only one here, so it should be fine.

BUG=aomedia:912

Change-Id: I6d75803e57da090db59eedad902bd27908eb5118