Auto merge of rust-lang#3769 - primoly:miri-start, r=RalfJung

Add `miri_start` support

This PR uses a function with the exported symbol `miri_start` as a drop-in alternative to `#[start]`. So the signature stays the same as suggested in [this comment](rust-lang/miri#3498 (comment)). <del>I’ve also removed Miri’s restriction  to only work on bin crates as I don’t think this is necessary anymore.</del>

Closes rust-lang#3758

Author: bors