handle visionos target OS tag in the compiler

* rename .xros to .visionos as agreed in the tracking issue
* add support for VisionOS platform in the MachO linker

lib/std/zig/system/darwin.zig

@@ -45,6 +45,7 @@ pub fn getSdk(allocator: Allocator, target: Target) ?[]const u8 {
         },
         .watchos => if (is_simulator_abi) "watchsimulator" else "watchos",
         .tvos => if (is_simulator_abi) "appletvsimulator" else "appletvos",
+        .visionos => if (is_simulator_abi) "xrsimulator" else "xros",
         else => return null,
     };
     const argv = &[_][]const u8{ "/usr/bin/xcrun", "--sdk", sdk, "--show-sdk-path" };