Rollup merge of #138175 - sam-mccall:binobj, r=nnethercote

Support rmeta inputs for --crate-type=bin --emit=obj

This already works for --emit=metadata, but is possible anytime we're not linking.

Tests:
- `rmeta_bin` checks we're not changing --emit=link (already passes)
- `rmeta_bin-pass` tests the new behavior for --emit=obj (would fail today) and also --emit=metadata which isn't changing

Author: Manishearth