Implement C _FloatN, _FloatNx types [version 2]

[FX]

> Fortran note: the float128_type_node used in the Fortran front end is
> renamed to gfc_float128_type_node, since the semantics are different:
> in particular, if long double has binary128 format, then the new
> language-independent float128_type_node is a distinct type that also
> has binary128 format, but the Fortran node is expected to be NULL in
> that case.  Likewise, Fortran's complex_float128_type_node is renamed
> to gfc_complex_float128_type_node.

Fortran part is OK.