go: introduce support for "go build" with gccgo.

The use of gccgo is triggered by GC=gccgo in environment. It
still needs the standard distribution to behave properly, but
allows using the test, build, run, install subcommands with
gccgo.

R=rsc, iant, fullung
CC=golang-dev, remy
https://golang.org/cl/5562045

src/cmd/go/pkg.go

@@ -270,6 +270,16 @@ func scanPackage(ctxt *build.Context, t *build.Tree, arg, importPath, dir string
 		p.target = filepath.Join(t.PkgDir(), filepath.FromSlash(importPath)+".a")
 	}
 
+	// For gccgo, rewrite p.target with the expected library name. We won't do
+	// that for the standard library for the moment.
+	if !p.Standard {
+		dir := t.PkgDir()
+		if _, ok := buildToolchain.(gccgoToolchain); ok {
+			dir = filepath.Join(filepath.Dir(dir), "gccgo", filepath.Base(dir))
+		}
+		p.target = buildToolchain.pkgpath(dir, p)
+	}
+
 	var built time.Time
 	if fi, err := os.Stat(p.target); err == nil {
 		built = fi.ModTime()

src/cmd/go/test.go

@@ -330,8 +330,7 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 	// We write the external test package archive to
 	// $WORK/unicode/utf8/_test/unicode/utf8_test.a.
 	testDir := filepath.Join(b.work, filepath.FromSlash(p.ImportPath+"/_test"))
-	ptestObj := filepath.Join(testDir, filepath.FromSlash(p.ImportPath+".a"))
-	pxtestObj := filepath.Join(testDir, filepath.FromSlash(p.ImportPath+"_test.a"))
+	ptestObj := buildToolchain.pkgpath(testDir, p)
 
 	// Create the directory for the .a files.
 	ptestDir, _ := filepath.Split(ptestObj)
@@ -380,8 +379,8 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 		pxtest.imports = append(pxtest.imports, ptest)
 		a := b.action(modeBuild, modeBuild, pxtest)
 		a.objdir = testDir + string(filepath.Separator)
-		a.objpkg = pxtestObj
-		a.target = pxtestObj
+		a.objpkg = buildToolchain.pkgpath(testDir, pxtest)
+		a.target = a.objpkg
 	}
 
 	// Action for building test.out.