Convert Type.Broke from a uint8 to a boolean

This change passes go build -toolexec 'toolstash -cmp' -a std.

Change-Id: I44e7548c71b00467e36576bdf62933c42555a21a
Reviewed-on: https://go-review.googlesource.com/14307
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Håvard Haugen <havard.haugen@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>