The asm macros are all set up now, introduce entry points.

GCM_INIT and GCM_COMPLETE have arguments supplied, so that
the new scatter/gather entry points don't have to take all the
arguments, and only the ones they need.
Signed-off-by: Dave Watson <davejwatson@fb.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>