Add visionOS support

[madsmtm]

I haven't added this to CI, as the Rust builds themselves are still somewhat broken until rust-lang/libc#3568 lands.

CC @BlackHoleFox

[QuentinPerez]

We were definitely working on the same thing; I made almost the same changes. The only differences are the XROS_DEPLOYMENT_TARGET and this additional check on lines 2658 and 2674

                // there is no -m{}os-version-min for watchos, tvos, visionos
                if matches!(os, AppleOs::Ios | AppleOs::MacOs) {
                    cmd.args.push(
                        format!("-m{}os-version-min={}", sdk_details.sdk_prefix, min_version)
                            .into(),
                    );
                }

[madsmtm]

there is no -m{}os-version-min for watchos, tvos, visionos

There is, see Clang's docs for the availability attribute and Clang's docs for e.g. the -mtvos-version-min flag itself.

However, you right that it does not exist for visionOS, I've opened llvm/llvm-project#88271 for that. This is somewhat problematic, as we need to tell Clang about the desired deployment target. Perhaps we can set XROS_DEPLOYMENT_TARGET? Or maybe -mtargetos=xros1.0?

[madsmtm]

Using just --target is recommended by Clang Driver developers, which we already do, so I think that is a non-issue.

Have opened #1030 to track removing the other flags from Apple targets.

[BlackHoleFox]

Nice :D

[NobodyXu]

Thank you!