Prepare for upcoming shared makefile change
RiscOS/BuildSys!62 will change how top-level phony rules are overridden.
In this component, the export, install, install_rom and rom targets
are redefined after AAsmModule is included, so they will be affected.
Rewrite the rules so that they will continue to work the same (at least for
native builds) both before and after BuildSys is updated.
!NoChange