Instead of comparing the address of the end of the memory to zero/copy,
comparing the address of the last element, which is a valid pointer.
Also unify large and unaligned Zero/Move, by passing alignment as AuxInt.

Fixes #16515 for ARM.

Change-Id: I19a62b31c5acf5c55c16a89bea1039c926dc91e5
Reviewed-on: https://go-review.googlesource.com/25300
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>