Today, signal.Ignore(syscall.SIGTRAP) does nothing
while signal.Notify(make(chan os.Signal), syscall.SIGTRAP)
correctly discards user-generated SIGTRAPs.
The same applies to any signal that we throw on.

Make signal.Ignore work for these signals.

Fixes #12906.

Change-Id: Iba244813051e0ce23fa32fbad3e3fa596a941094
Reviewed-on: https://go-review.googlesource.com/18348Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>