mkdir rt fails because it exists

https://github.com/mozilla/rust/blob/master/configure#L748

```
mkdir: error: Error: directory 'i686-pc-mingw32/rt' already exists
```

``` shell
for t in $CFG_TARGET
do
  make_dir $t/rt
  for s in 0 1 2 3
  do
```
