[dev.ssa] cmd/compile/internal/gc: avoid generating test binary file

[dev.ssa] cmd/compile/internal/gc: avoid generating test binary file

Using the main package causes a binary to be generated.  That binary
clutters up git listings.

Use a non-main package instead, so the results of a successful
compilation are thrown away.

Change-Id: I3ac91fd69ad297a5c0fe035c22fdef290b7dfbc4
Reviewed-on: https://go-review.googlesource.com/14399
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>