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* Define `DEFMOD` for cross-compilation use.
* Define `SETTYPE` for cross-compilation use (this acts as a no-op; when
  cross-compiling, filetypes are indicated using ,xxx suffixes instead).
* Define `RUN` tool for both native and cross-compilation. This is already
  used in various components, but wasn't defined anywhere. It is particularly
  useful during cross-compilation for launching build tools that were built
  locally to a component, since on systems other than RISC OS, the current
  directory is typically not on the run path.