Update prebuilts to go1.9rc1 ab/4215840

Test: m -j blueprint_tools
Change-Id: I6e92d224c7b1185c0813593ab11403ef50017916
diff --git a/src/os/stat_unix.go b/src/os/stat_unix.go
index 1733d3f..1dd5714 100644
--- a/src/os/stat_unix.go
+++ b/src/os/stat_unix.go
@@ -17,7 +17,7 @@
 		return nil, ErrInvalid
 	}
 	var fs fileStat
-	err := syscall.Fstat(f.fd, &fs.sys)
+	err := f.pfd.Fstat(&fs.sys)
 	if err != nil {
 		return nil, &PathError{"stat", f.name, err}
 	}