Fortran 2018 added some synonyms to the existing values, namely
IEEE_NEGATIVE_SUBNORMAL (which is the same as IEEE_NEGATIVE_DENORMAL)
and IEEE_POSITIVE_SUBNORMAL (same as IEEE_POSITIVE_DENORMAL). When they
were added to the C side, they were not kept in sync with the Fortran
part of the library. Thew new values are not used (yet), so it is
currently harmless, but better fix it.

libgfortran/ChangeLog:

	* ieee/ieee_helper.c: Fix enum values.