mruby: Fixes build on Linux

I'm not sure of the semantics of `gcc` vs. `ld` for linking, but it
makes the difference between working and:

```
/nix/store/gpc2wld1s0c6qzx9326cwn1wcx29xzsj-binutils-2.30/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000401100
```