runtime: add explicit (void) in C to avoid GCC 7 problem

This avoids errors like
    ./traceback.go:80:2: call of non-function C.f1

I filed https://gcc.gnu.org/PR79289 for the GCC problem. I think this
is a bug in GCC, and it may be fixed before the final GCC 7 release.
This CL is correct either way.

Fixes #18855.

Change-Id: I0785a7b7c5b1d0ca87b454b5eca9079f390fcbd4
Reviewed-on: https://go-review.googlesource.com/35919


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>