sparc: rename piggyback_32 to piggyback

Now that we use the same piggyback for 32 and 64 bit
we can drop the _32 suffix.
Include some trivial unification in the Makefile
now that 32 and 64 bit can share the same piggyback command.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>