I guess the idea would be to eventually also install all of the
openlibm*.h headers, instead of just openlibm.h. Make openlibm_fenv.h
suitable for this purpose by moving all of the $ARCH/fenv.h headers next
to it.

We actually need this change to make OPENLIBM_USE_HOST_FENV_H work.
Right now it's still broken, because the "#include <fenv.h>" performed
by openlibm_fenv.h still pulls in $ARCH/fenv.h as $ARCH/ is added to the
compiler include path.