Declare wcstofN, wcstofNx for C2x

WG14 accepted the changes in N3105 to define wcstofN and wcstofNx
functions for C2x.  Thus enable those for C2x (given also __GLIBC_USE
(IEC_60559_TYPES_EXT) and support for the relevant _FloatN / _FloatNx
type) rather than only for __USE_GNU.

Tested for x86_64.