runtime: preserve callee-saved C registers in sigtramp
This fixes Linux and the *BSD platforms on 386/amd64.
A few OS/arch combinations were already saving registers and/or doing
something that doesn't clearly resemble the SysV C ABI; those have
been left alone.
Fixes #18328.
Change-Id: I6398f6c71020de108fc8b26ca5946f0ba0258667
Reviewed-on: https://go-review.googlesource.com/34501
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>