cmd/internal/obj/arm64: provide and use an oprrrr function

cmd/internal/obj/arm64: provide and use an oprrrr function

Provide a four register version of oprrr, which takes an additional 'ra'
register. Use this instead of oprrr where appropriate.

Change-Id: I8882957a83c2b08e407f37a37c61864cd920bbc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/471519
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>