runtime: fix panic stack during runtime.Goexit during panic

A runtime.Goexit during a panic-invoked deferred call
left the panic stack intact even though all the stack frames
are gone when the goroutine is torn down.
The next goroutine to reuse that struct will have a
bogus panic stack and can cause the traceback routines
to walk into garbage.

Most likely to happen during tests, because t.Fatal might
be called during a deferred func and uses runtime.Goexit.

This "not enough cleared in Goexit" failure mode has
happened to us multiple times now. Clear all the pointers
that don't make sense to keep, not just gp->panic.

Fixes issue 8158.

LGTM=iant, dvyukov
R=iant, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/102220043