cmd/gc: fix handling of struct padding in hash/eq.

cmd/gc: fix handling of struct padding in hash/eq.

The test case of issue 4585 was not passing due to
miscalculation of memequal args, and the previous fix
does not handle padding at the end of a struct.

Handling of padding at end of structs also fixes the case
of [n]T where T is such a padded struct.

Fixes #4585.
(again)

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7133059