Auto merge of #3071 - japaric:moar-cargo, r=alexcrichton

prepare for cargo for mips and powerpc

with these changes I can cross compile Cargo for these targets:

- mips-unknown-linux-gnu
- mipsel-unknown-linux-gnu
- powerpc-unknown-linux-gnu
- powerpc64-unknown-linux-gnu
- powerpc64el-unknown-linux-gnu

using these commands:

```
$ ./.travis.install.deps.sh
$ ./configure --local-rust-root=$(pwd)/rustc --enable-nightly --target=$TARGET
$ make
```

in Ubuntu 16.04 using the standard cross toolchains

r? @alexcrichton
cross compiling Cargo for the mips targets requires rust-lang/libc#378