[release-branch.go1.5] runtime: improve stack barrier debugging
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/17147
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/17193
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>