exp/ssa: perform all packages' BUILD phases in parallel.

Details:
- move Builder.nTo1Vars into package => thread-safe.
- add BuildSerially builder mode flag to disable concurrency.
- add Builder.BuildAllPackages method.

Benchmark: BuildAllPackages for $GOROOT/test/append.go drops
to 83ms from 190ms (GOMAXPROCS=8).

R=gri
CC=golang-dev
https://golang.org/cl/7371051

src/pkg/exp/ssa/interp/interp_test.go

@@ -182,7 +182,7 @@ func run(t *testing.T, dir, input string) bool {
 		return false
 	}
 
-	b.BuildPackage(mainpkg)
+	b.BuildAllPackages()
 	b = nil // discard Builder
 
 	hint = fmt.Sprintf("To trace execution, run:\n%% go run exp/ssa/ssadump.go -build=C -run --interp=T %s\n", input)

src/pkg/exp/ssa/ssa.go

@@ -45,7 +45,9 @@ type Package struct {
 
 	// The following fields are set transiently during building,
 	// then cleared.
-	files []*ast.File // the abstract syntax tree for the files of the package
+	started  int32                   // atomically tested and set at start of build phase
+	files    []*ast.File             // the abstract syntax trees for the files of the package
+	nTo1Vars map[*ast.ValueSpec]bool // set of n:1 ValueSpecs already built
 }
 
 // A Member is a member of a Go package, implemented by *Literal,

src/pkg/exp/ssa/ssadump.go

@@ -23,6 +23,7 @@ P	log [P]ackage inventory.
 F	log [F]unction SSA code.
 S	log [S]ource locations as SSA builder progresses.
 G	use binary object files from gc to provide imports (no code).
+L	build distinct packages seria[L]ly instead of in parallel.
 N	build [N]aive SSA form: don't replace local loads/stores with registers.
 `)
 
@@ -66,6 +67,8 @@ func main() {
 			mode |= ssa.NaiveForm
 		case 'G':
 			mode |= ssa.UseGCImporter
+		case 'L':
+			mode |= ssa.BuildSerially
 		default:
 			log.Fatalf("Unknown -build option: '%c'.", c)
 		}
@@ -128,7 +131,7 @@ func main() {
 	if err != nil {
 		log.Fatalf(err.Error())
 	}
-	b.BuildPackage(mainpkg)
+	b.BuildAllPackages()
 	b = nil // discard Builder
 
 	if *runFlag {