powerpc: Fix Handler of Unaligned Load/Store Strings

The alignment handler is incorrect for unaligned string instructions
in little endian mode.  These instructions access data as arrays of
bytes and thus are endian neutral.  However, the routine also handles
the load/store multiple instructions, which are NOT endian neutral.

This patch toggles the byte swapping flag for the string instructions
in little endian builds.  This effectively disables the byte swapping
logic.

Signed-off-by: Tom Musta <tmusta@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>