runtime: use indexed load/store in ARM64 assembly

Minor optimization. Spotted while working on that code.

Change-Id: Ia02dee10d74bce79a0bef1eaba7fac1bfc27df38
Reviewed-on: https://go-review.googlesource.com/c/go/+/266899


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: fannie zhang <Fannie.Zhang@arm.com>
Reviewed-by: David Chase <drchase@google.com>