runtime: check explicitly for short unwinding of stacks

runtime: check explicitly for short unwinding of stacks

Right now we find out implicitly if stack barriers are in place,
or defers. This change makes sure we find out about short
unwinds always.

Change-Id: Ibdde1ba9c79eb792660dcb7aa6f186e4e4d559b3
Reviewed-on: https://go-review.googlesource.com/13966
Reviewed-by: Austin Clements <austin@google.com>