cmd/internal/obj/arm64: factor out constant classification code

cmd/internal/obj/arm64: factor out constant classification code

This will allow for further improvements and deduplication.

Change-Id: I9374fc2d16168ced06f3fcc9e558a9c85e24fd01
Reviewed-on: https://go-review.googlesource.com/c/go/+/650936
Reviewed-by: Fannie Zhang <Fannie.Zhang@arm.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>