cmd/go: fix data race during build

Fixes #2695.

R=golang-dev, mpimenov, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/5545052

src/cmd/go/build.go

@@ -873,6 +873,8 @@ func (b *builder) runOut(dir string, desc string, cmdargs ...interface{}) ([]byt
 
 // mkdir makes the named directory.
 func (b *builder) mkdir(dir string) error {
+	b.exec.Lock()
+	defer b.exec.Unlock()
 	// We can be a little aggressive about being
 	// sure directories exist.  Skip repeated calls.
 	if b.mkdirCache[dir] {