MIPS: Unify memcpy.S and memcpy-inatomic.S

We can save the 451 lines of code that comprise memcpy-inatomic.S at the
expense of a single instruction in the memcpy prolog.  We also use an
additional register (t6), so this may cause increased register pressure in
some places as well.  But I think the reduced maintenance burden, of not
having two nearly identical implementations, makes it worth it.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>