[dev.typeparams] cmd/compile: add a field (method) name for function in TestABIUtilsInterfaces

Not having a field name for the method is not really correct, and makes
it look like an embedded field. In fact, currently types.CalcSize() in
abitest() is creating an error that is not actually reported.

Change-Id: I98c3a4abf5b6d610d9c3c56ce1042078374b5417
Reviewed-on: https://go-review.googlesource.com/c/go/+/331469


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>