make.bat: Fix for old files

Same fix as applied here: https://golang.org/cl/5761044

Fixes #3222.

R=golang-dev, alex.brainman, rsc
CC=golang-dev
https://golang.org/cl/5838043

src/make.bat

@@ -28,6 +28,9 @@ cd src
 if "x%GOROOT_FINAL%"=="x" set GOROOT_FINAL=%GOROOT%
 set DEFGOROOT=-DGOROOT_FINAL="\"%GOROOT_FINAL:\=\\%\""
 
+:: Clean old generated file that will cause problems in the build.
+del /F ".\pkg\runtime\runtime_defs.go"
+
 echo # Building C bootstrap tool.
 echo cmd/dist
 if not exist ..\bin\tool mkdir ..\bin\tool