Previously, when a program died because of a SIGHUP, SIGINT, or SIGTERM
signal it would exit with status 2.  This CL fixes the runtime to exit
with a status indicating that the program was killed by a signal.

Change-Id: Ic2982a2562857edfdccaf68856e0e4df532af136
Reviewed-on: https://go-review.googlesource.com/18156Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>