On macos, allow targets supported by the SDK. This then spawns `xcrun`
and correct paths are emitted for:

- x86_64-macos
- x86_64-ios
- x86_64-tvos
- x86_64-watchos
- x86_64-ios-macbi
- aarch64-macos
- aarch64-ios
- aarch64-tvos
- aarch64-watchos
- aarch64-ios-macbi

On platforms with android NDK, allow android targets. Example usage:

```
CC=/NDK/.../bin/aarch64-linux-android34-clang zig libc -target aarch64-linux-android
```