[dev.regabi] cmd/compile: setup to move Addrconst, Patch into cmd/internal/obj

Deleting the Pc assignment from Patch is safe because the actual PCs
are not assigned until well after the compiler is done patching jumps.
And it proves that replacing uses of Patch with SetTarget will be safe later.

Change-Id: Iffcbe03f0b5949ccd4c91e79c1272cd06be0f434
Reviewed-on: https://go-review.googlesource.com/c/go/+/279296


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>