runtime: fix use of wrong g in gentraceback

runtime: fix use of wrong g in gentraceback

gentraceback gets the currently running g to do some sanity checks, but
should use gp everywhere to do its actual work. Some noncritical checks
later accidentally used g instead of gp. This seems like it could be a
problem in many different contexts, but I noticed in Windows profiling,
where profilem calls gentraceback on a goroutine from a different
thread.

Change-Id: I3da27a43e833b257f6411ee6893bdece45a9323f
Reviewed-on: https://go-review.googlesource.com/128895
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Austin Clements <austin@google.com>