[release-branch.go1.15] cmd/compile, runtime: store pointers to go:notinheap types indirectly

pointers to go:notinheap types should be treated as scalars. That
means they shouldn't be stored directly in interfaces, or directly
in reflect.Value.ptr.

Also be sure to use uintpr to compare such pointers in reflect.DeepEqual.

Fixes #42169

Change-Id: I53735f6d434e9c3108d4940bd1bae14c61ef2a74
Reviewed-on: https://go-review.googlesource.com/c/go/+/264480


Trust: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit 009d7140)
Reviewed-on: https://go-review.googlesource.com/c/go/+/265720
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>