runtime: change heapObjectsCanMove to a func

runtime: change heapObjectsCanMove to a func

A var is problematic because the zero value is already false,
so if it goes away, it will appear to be false.
I'm also not sure about go:linkname on vars,
so switch to func for both reasons.

Also add a test.

Change-Id: I2318a5390d98577aec025152e65543491489defb
Reviewed-on: https://go-review.googlesource.com/c/go/+/498261
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>