native xmake toolchain definition for UE-clang (#805)

This change is meant to provide a smoother experience when working on Linux. After this change, the toolchain setup process is now simply

```bash
$ scripts/ue_build_linux/get_ue_toolchain.sh 
```

and then at config time the toolchain is automatically detected if you downloaded it to the default location or have the `UE_TOOLCHAIN_DIR` environment variable set

```bash
xmake config --mode=debug
```

Compared to the old script-based approach this configures the toolchain more precisely, avoiding leakage into unrelated build processes such as when a package manager decides to build something like Ninja locally etc.

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