Declare wcstofN, wcstofNx for C2x

[Andreas Schwab]

On Feb 25 2023, Joseph Myers wrote:

> 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.

Ok.

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Andreas Schwab, SUSE Labs, schwab@suse.de
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