sh: Provide optimized unaligned loads on SH-4A.

This adds support for unaligned loads on SH-4A, using the SH-4A's
neutered movua.l instruction. As movua.l is r0-inspired, stores are
still handled through the packed struct.

Based on asm-generic/unaligned.h by Harvey Harrison.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>