[release-branch.go1.15] cmd/compile: export notinheap annotation to object file

In the rare case when a cgo type makes it into an object file, we need
the go:notinheap annotation to go with it.

Fixes #41432.

Change-Id: Ie2ef241ee49661792e0d8c8c46c51b2fe5c6fa7c
Reviewed-on: https://go-review.googlesource.com/c/go/+/259300


Trust: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>