This CL extends checkptrBase to recognize pointers into the stack and
data/bss sections. I was meaning to do this eventually anyway, but
it's also an easy way to workaround #35068.

Updates #35068.

Change-Id: Ib47f0aa800473a4fbc249da52ff03bec32c3ebe2
Reviewed-on: https://go-review.googlesource.com/c/go/+/202639
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>