runtime: darwin/amd64, don't depend on outarg slots being unmodified

sigtramp was calling sigtrampgo and depending on the fact that
the 3rd argument slot will not be modified on return.  Our calling
convention doesn't guarantee that.  Avoid that assumption.

There's no actual bug here, as sigtrampgo does not in fact modify its
argument slots.  But I found this while working on the dead stack slot
clobbering tool.  https://go-review.googlesource.com/c/23924/

Change-Id: Ia7e791a2b4c1c74fff24cba8169e7840b4b06ffc
Reviewed-on: https://go-review.googlesource.com/36216


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>