CL 57291 broke on solaris because it depends on signal forwarding
working for signals raised by dieFromSignal.
Call sigtrampgo instead of sighandler directly, like the other
unix platforms.

Fixes the solaris builders.

Change-Id: I6bf314c436d1edeaecc4b03f15a9155270919524
Reviewed-on: https://go-review.googlesource.com/59811
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>