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/13966Reviewed-by: Austin Clements <austin@google.com>