There's no need to call/ret to the body implementation.
It can write the result to the right place.  Just jump to
it and have it return to our caller.

Old:
  call body implementation
  compute result
  put result in a register
  return
  write register to result location
  return

New:
  load address of result location into a register
  jump to body implementation
  compute result
  write result to passed-in address
  return

It's a bit tricky on 386 because there is no free register
with which to pass the result location.  Free up a register
by keeping around blen-alen instead of both alen and blen.

Change-Id: If2cf0682a5bf1cc592bdda7c126ed4eee8944fba
Reviewed-on: https://go-review.googlesource.com/9202Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>