cmd/go: for gccgo, don't edit cgo header when using -o

This change was made to the gccgo sources as part of CL 47037.
It is required to make the testcarchive and testcshared tests work.
Otherwise using `go build -mode=c-archive -o libgo.a` will cause the
header file to be named go.h rather than libgo.h.

Change-Id: I2db1d7b0f575368b31273cc01097447a0471efd6
Reviewed-on: https://go-review.googlesource.com/111615
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/go/internal/work/action.go

@@ -588,7 +588,7 @@ func (b *Builder) addInstallHeaderAction(a *Action) {
 	p := a.Package
 	if p.UsesCgo() && (cfg.BuildBuildmode == "c-archive" || cfg.BuildBuildmode == "c-shared") {
 		hdrTarget := a.Target[:len(a.Target)-len(filepath.Ext(a.Target))] + ".h"
-		if cfg.BuildContext.Compiler == "gccgo" {
+		if cfg.BuildContext.Compiler == "gccgo" && cfg.BuildO == "" {
 			// For the header file, remove the "lib"
 			// added by go/build, so we generate pkg.h
 			// rather than libpkg.h.