sh: Add register alignment helpers for shared flushers.

This plugs in some register alignment helpers for the shared flushers,
allowing them to also be used on SH-5. The main rationale here is that
in the SH-5 case we have a variable ABI, where the pointer size may not
equal the register width. This register extension is taken care of by
the SH-5 code already today, and is otherwise unused on the SH-4 code.
This combines the two and allows us to kill off the SH-5 implementation.

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