We weren't setting r0 to 0, as required by our generated code.

Before this patch, the misc/cgo/testcarchive tests failed on ppc64le.
After this patch, they work, so enable them.

Change-Id: I53b16746961da9f7c34f59030a1e40953c9c1e05
Reviewed-on: https://go-review.googlesource.com/44093
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>