rustbuild: Fix recompilation of stage0 tools dir

This commit knocks out a longstanding FIXME in rustbuild which should correctly
recompile stage0 compiletest and such whenever libstd itself changes. The
solution implemented here was to implement a notion of "order only" dependencies
and then add a new dependency stage for clearing out the tools dir, using
order-only deps to ensure that it happens correctly.

The dependency drawing for tools is a bit wonky now but I think this'll get the
job done.

Closes #39396

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