Adding GNU toolchain for the msp430-elf architecture.

Don't know how often you guys accept crosscompilers, but here goes. I've noticed is that on a machine with only `gcc` installed the link step tries to overwrite `/usr/local/lib/gcc`. If the machine has more than one `gcc` installed (mine had `gcc49`, and `gcc48`) this does not seem to be an issue. The workaround (found by @durka) was as follows:

```sh
brew unlink gcc
mkdir /usr/local/lib/gcc
brew link gcc
brew link msp430-elf-gcc
```

I'd love to resolve that. Any advice is greatly appreciated. Thanks!