This improves stack barrier debugging messages in various ways:

1) Rather than printing only the remaining stack barriers (of which
   there may be none, which isn't very useful), print all of the G's
   stack barriers with a marker at the position the stack itself has
   unwound to and a marker at the problematic stack barrier (where
   applicable).

2) Rather than crashing if we encounter a stack barrier when there are
   no more stkbar entries, print the same debug message we would if we
   had encountered a stack barrier at an unexpected location.

Hopefully this will help with debugging #12528.

Change-Id: I2e6fe6a778e0d36dd8ef30afd4c33d5d94731262
Reviewed-on: https://go-review.googlesource.com/17147Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>