Auto merge of #1947 - RalfJung:macos-is-from-the-stone-age, r=RalfJung
Auto merge of #1947 - RalfJung:macos-is-from-the-stone-age, r=RalfJung

macOS-compatible realpath

Make behavior consistent between Linux and macOS even though macOS lacks some basic shell tools

rust-lang/miri,rust-lang/miri,rust-lang/miri,rust-lang/miri