Currently the stack barrier stub blindly unwinds the next stack
barrier from the G's stack barrier array without checking that it's
the right stack barrier. If through some bug the stack barrier array
position gets out of sync with where we actually are on the stack,
this could return to the wrong PC, which would lead to difficult to
debug crashes. To address this, this commit adds a check to the amd64
stack barrier stub that it's unwinding the correct stack barrier.

Updates #12238.

Change-Id: If824d95191d07e2512dc5dba0d9978cfd9f54e02
Reviewed-on: https://go-review.googlesource.com/13948Reviewed-by: Russ Cox <rsc@golang.org>