[release-branch.go1.15] cmd/compile: make go:notinheap error message friendlier for cgo

Update #40954

Change-Id: Ifaab7349631ccb12fc892882bbdf7f0ebf3d845f
Reviewed-on: https://go-review.googlesource.com/c/go/+/251158


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Keith Randall <khr@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/255338


Reviewed-by: Austin Clements <austin@google.com>