cmd/go: support -compiler for go list, fix isStale for gccgo.

Fixes #3228.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5784044

src/cmd/go/list.go

@@ -86,6 +86,7 @@ For more about specifying packages, see 'go help packages'.
 
 func init() {
 	cmdList.Run = runList // break init cycle
+	cmdList.Flag.Var(buildCompiler{}, "compiler", "")
 }
 
 var listE = cmdList.Flag.Bool("e", false, "")

src/cmd/go/pkg.go

@@ -372,7 +372,7 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 	}
 
 	// unsafe is a fake package.
-	if p.Standard && p.ImportPath == "unsafe" {
+	if p.Standard && (p.ImportPath == "unsafe" || buildContext.Compiler == "gccgo") {
 		p.target = ""
 	}
 
@@ -417,7 +417,7 @@ func computeStale(pkgs ...*Package) {
 
 // isStale reports whether package p needs to be rebuilt.
 func isStale(p *Package, topRoot map[string]bool) bool {
-	if p.Standard && p.ImportPath == "unsafe" {
+	if p.Standard && (p.ImportPath == "unsafe" || buildContext.Compiler == "gccgo") {
 		// fake, builtin package
 		return false
 	}