build: support multiple output directories

[MaskRay]

madscientist has a nice essay on this: http://make.mad-scientist.net/papers/multi-architecture-builds/ if you want to stay the pure GNU make route.

[rui314]

I'm not sure if we want to support multiple output directories. That's good to have, but since mold is a small project, you may be able to live with just a single output directory. I believe we eventually have to migrate to CMake or something, but before that, I'd like to stick with simple Makefile rules unless there's a special reason to change that.