[dev.regabi] cmd/compile: refactor abiutils from "gc" into new "abi"

Needs to be visible to ssagen, and might as well start clean to avoid
creating a lot of accidental dependencies.

Added some methods for export.

Decided to use a pointer instead of value for ABIConfig uses.

Tests ended up separate from abiutil itself; otherwise there are import cycles.

Change-Id: I5570e1e6a463e303c5e2dc84e8dd4125e7c1adcc
Reviewed-on: https://go-review.googlesource.com/c/go/+/282614


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>
Reviewed-by: Jeremy Faller <jeremy@golang.org>