duplicate ARM targets in `rustc --print target-list`

### STR

```
$ rustc --print target-list | uniq -c | grep -v 1
      2 arm-unknown-linux-gnueabi
      2 arm-unknown-linux-gnueabihf
      2 armv7-unknown-linux-gnueabihf
```
### Meta

```
$ rustc -V
rustc 1.12.0-nightly (b30eff7ba 2016-08-05)
```
