2d637f70a8
makes entry_abi a lowering of the ABI string, so now it can be ```json "entry_abi": "C", "entry_abi": "win64", "entry_abi": "aapcs", ```
rustc_target contains some very low-level details that are
specific to different compilation targets and so forth.
For more information about how rustc works, see the rustc dev guide.