internal/runtime/maps: assume constant elem offset with int64 and string keys

internal/runtime/maps: assume constant elem offset with int64 and string keys

Note this doesn't work with int32 keys because alignment padding can change
the offset of the element.

Change-Id: I27804d3cfc7cc1b7f995f7e29630f0824f0ee899
Reviewed-on: https://go-review.googlesource.com/c/go/+/629418
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>