Propagate the return code from the `start` lang item
Propagate the return code from the `start` lang item

Fixes #1064

This ensures that we set the error code properly when a panic unwinds
past `main`.

I'm not sure what the best way to write a test for this is

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