Add rustc-ap-rustc_target to dependencies

[topecongiro]

Recently abi module was remove from libsyntax and moved to librustc_target (cc rust-lang/rust#50228). In order to catch up with the change, we need to:

1. Update rustc-ap_syntax and add rustc-ap-rustc_target to depenencies.
2. Replace syntax::abi with rustc_target::spec::abi.
3. Fix broken code, if available.