runtime: accept GOTRACEBACK=crash to mean 'crash after panic'

This provides a way to generate core dumps when people need them.
The settings are:

        GOTRACEBACK=0  no traceback on panic, just exit
        GOTRACEBACK=1  default - traceback on panic, then exit
        GOTRACEBACK=2  traceback including runtime frames on panic, then exit
        GOTRACEBACK=crash traceback including runtime frames on panic, then crash

Fixes issue 3257.

R=golang-dev, devon.odell, r, daniel.morsing, ality
CC=golang-dev
https://codereview.appspot.com/7666044