sh: Tidy up SH-4A unaligned load support.

The current implementation was rather tied to the packed_struct.h
definitions, which immediately began to clash when the packed_struct.h
types changed and drivers began to include packed_struct.h directly.

In order to support this sort of use it's necessary to get out of the way
with regards to namespace collisions, and at the same time we can also
kill off some duplicate code now that the unaligned headers are a bit
more broken out.

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