add_debug_reloc emits RelocationKind::Relative + RelocationEncoding::Generic + non-zero addend relocation, which isn't implemented for AArch64 Mach-O

[cbeuw]

Part of #1248.

To reproduce, simply y.rs build on an Apple Silicon machine. Sysroot build fails with

error: error writing object file: unimplemented relocation Relocation { offset: 32, size: 32, kind: Relative, encoding: Generic, symbol: SymbolId(22), addend: -4 }

This relocation is emitted here
https://github.com./bjorn3/rustc_codegen_cranelift/blob/215dd7aa0d70cf7396100715326a37c81b06f056/src/debuginfo/object.rs#L69-L81

This is unimplemented in gimli-rs.

I'm unfamiliar with executable formats, so not sure if this should be implemented in gimli-rs or make rustc_codegen_cranelift emit something else.

[bjorn3]

See also https://github.com./bjorn3/rustc_codegen_cranelift/issues/1248. I will add this issue to the list.

[bjorn3]

This particular issue should now be fixed, but arm64 macOS support is still blocked on vararg support.