cmd/go: fix handling of gccgo standard library.

The previous logic was mainly non-working. It only needs to
ensure that the go tool doesn't try to build the standard
library with gccgo.

R=golang-dev, rsc
CC=golang-dev, remy
https://golang.org/cl/5580051

src/cmd/go/build.go

@@ -349,6 +349,12 @@ func (b *builder) action(mode buildMode, depMode buildMode, p *Package) *action
 			// Fake packages - nothing to build.
 			return a
 		}
+		// gccgo standard library is "fake" too.
+		if _, ok := buildToolchain.(gccgoToolchain); ok {
+			// the target name is needed for cgo.
+			a.target = p.target
+			return a
+		}
 	}
 
 	if !p.Stale && !buildA && p.target != "" {

src/cmd/go/pkg.go

@@ -295,14 +295,6 @@ func scanPackage(ctxt *build.Context, t *build.Tree, arg, importPath, dir string
 			dir = filepath.Join(filepath.Dir(dir), "gccgo", filepath.Base(dir))
 		}
 		p.target = buildToolchain.pkgpath(dir, p)
-
-		// NB. Currently we have gccgo install the standard libraries
-		// in the "usual" location, where the Go toolchain puts them.
-		if p.Standard {
-			if _, ok := buildToolchain.(gccgoToolchain); ok {
-				p.target = goToolchain{}.pkgpath(dir, p)
-			}
-		}
 	}
 
 	var built time.Time