Refactor static and dynamic handling to avoid hard-coding musl #542

With #541, musl is switching to dynamic linking and it became apparent that the build system special-cases musl specifically instead of tracking if the build is statically or dynamically linked. It'd be great to add an abstraction to capture the concept rather than checking the target triple.

It's unclear if we'll need this long-term, but it's not a lot of work anyway.