Add `visionos` (Apple VisionOS) to `apple_targets` when it is supported by rustc

[SteveLauC]

Apple has introduced their VisionOS, and thus we have one more target to add, developers at Apple are adding the support to the upstream LLVM compiler, when complete, the rustc team will also support it.

Upstream Issues

• LLVM: xros and visionos compilation support?
• rustc: Add support for visionOS targets

[SteveLauC]

libc PR: rust-lang/libc#3568