[dev.garbage] runtime: determine if an object is public

ROC (request oriented collector) needs to determine
if an object is visible to other goroutines, i.e.
public. In a later CL this will be used by the write
barrier and the publishing logic to distinguish between
local and public objects and act accordingly.

Change-Id: I6a80da9deb21f57e831a2ec04e41477f997a8c33
Reviewed-on: https://go-review.googlesource.com/25056


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