[dev.regabi] cmd/compile: modify abiutils for recently updated ABI

Discovered difficluties posed by earlier design, these modifications
should work better.  Updated tests, also added some helper functions
for use in call lowering.

Change-Id: I459f0f71ad8a6730c571244925c3f395e1df28de
Reviewed-on: https://go-review.googlesource.com/c/go/+/285392


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>