Auto merge of #3007 - whitequark:opt-level-s, r=alexcrichton

Allow using opt-level="s"/"z" in profile overrides

Initially, I've considered making a dedicated `OptLevel` enum, but this appeared to bring no practical benefit, only boilerplate, so I've used a String instead, which is also in line with the `u32` that was there before, not even checked for being in range `0...3`.