[MIPS] Unify memset.S

The 32-bit version and 64-bit version are almost equal.  Unify them.
This makes further improvements (for example, supporting CDEX, etc.)
easier.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>