Apply ABI attributes on return types in rustc_codegen_cranelift

[beetrees]

• The x86-64 System V ABI standard doesn't sign/zero-extend integer arguments or return types.
• But the de-facto standard as implemented by Clang and GCC is to sign/zero-extend arguments to 32 bits (but not return types).
• Additionally, Apple targets sign/zero-extend both arguments and return values to 32 bits.
• However, the rustc_target ABI adjustment code currently unconditionally extends both arguments and return values to 32 bits on all targets.
• This doesn't cause a miscompilation when compiling with LLVM as LLVM will ignore the signext/zeroext attribute when applied to return types on non-Apple x86-64 targets.
• Cranelift, however, does not have a similar special case, requiring rustc to set the argument extension attribute correctly.
• However, rustc_codegen_cranelift doesn't currently apply ABI attributes to return types at all, meaning rustc_codegen_cranelift will currently miscompile i8/u8/i16/u16 returns on x86-64 Apple targets as those targets require sign/zero-extension of return types.

This PR fixes the bug(s) by making the rustc_target x86-64 System V ABI only mark return types as sign/zero-extended on Apple platforms, while also making rustc_codegen_cranelift apply ABI attributes to return types. The RISC-V and s390x C ABIs also require sign/zero extension of return types, so this will fix those targets when building with rustc_codegen_cranelift too.

r? @bjorn3

[rustbot]

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

[bjorn3]

@bors2 try jobs=x86_64-apple-*

[rust-bors]

⌛ Trying commit eb472e7 with merge 6a4c445…

To cancel the try build, run the command @bors2 try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 6a4c445 (6a4c445322019216b3977c8391f73f8ba182585a, parent: 6c8138de8f1c96b2f66adbbc0e37c73525444750)

[bjorn3]

@bors r+

[bors]

📌 Commit eb472e7 has been approved by bjorn3

It is now in the queue for this repository.