cmd/compile/internal/test: update abi tests for recent spec changes

The internal ABI spec was recently updated to include specific
language covering "past-the-end" pointers and structs containing
trailing zero-sized fields. Add a unit test that makes sure we do the
right thing in this case. Fix a couple comments in other unit tests.

Change-Id: I18d373d11e122aec74b316837843887272676c63
Reviewed-on: https://go-review.googlesource.com/c/go/+/303809


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