Note: I found a bug in c-stack-cdecl which codes not permit such
native functions to be used as values. I added an xfail-test
(c-stack-as-value) documenting it.
While it is still technically possible to test stage 0, it is not part of any
of the main testing rules and maintaining xfail-stage0 is a chore. Nobody
should worry about how tests fare in stage0.