The c-archive test were recently converted from shell script to Go.
Unfortunately, it also lost the ability to target iOS and Android
that lack C compilers and require exec wrappers.

Compile the c-archive test for the host and run it with the target
GOOS/GOARCH environment. Change the test to rely on go env GOOS
and go env GOARCH instead of runtime.GOOS and runtime.GOARCH.

Fixes #8345

Change-Id: I290ace2f7e96b87c55d99492feb7d660140dcb32
Reviewed-on: https://go-review.googlesource.com/21102Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>