cmd/internal/obj/arm64: fix the size of STP series instructions in optab

When the class of p.To is C_NAUTO4K, STP series instructions will be translated
into add/sub + stp instructions, the total size is 8. Currently this size
value in optab is 12, this CL fixes it.

Change-Id: Ife1b9e9f8a28edca9b59f62b45a80d83f38993b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/314210


Reviewed-by: eric fang <eric.fang@arm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: eric fang <eric.fang@arm.com>
Run-TryBot: eric fang <eric.fang@arm.com>
TryBot-Result: Go Bot <gobot@golang.org>