[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/14399Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/testdata/deferNoReturn_ssa.go

@@ -7,7 +7,7 @@
 // Test that a defer in a function with no return
 // statement will compile correctly.
 
-package main
+package foo
 
 func deferNoReturn_ssa() {
 	defer func() { println("returned") }()
@@ -15,7 +15,3 @@ func deferNoReturn_ssa() {
 		println("loop")
 	}
 }
-
-func main() {
-	deferNoReturn_ssa()
-}