cmd/go: remove _cgo_flags from gccgo archive built using SWIG

Since SWIG uses cgo, when using SWIG with gccgo, the go tool will add a
_cgo_flags file to the package archive, just as it does with cgo. We
need to remove that file from the archive passed to the linker, just as
we do with cgo.

Change-Id: I5ef4fea92eec419f870fbf6f678691d15901ee6c
Reviewed-on: https://go-review.googlesource.com/32535
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>

src/cmd/go/build.go

@@ -2797,7 +2797,7 @@ func (tools gccgoToolchain) link(b *builder, root *action, out string, allaction
 			if !apackagePathsSeen[a.p.ImportPath] {
 				apackagePathsSeen[a.p.ImportPath] = true
 				target := a.target
-				if len(a.p.CgoFiles) > 0 {
+				if len(a.p.CgoFiles) > 0 || a.p.usesSwig() {
 					target, err = readAndRemoveCgoFlags(target)
 					if err != nil {
 						return