Implement C23 const-preserving standard library macros

[Andreas Schwab]

On Nov 18 2025, Joseph Myers wrote:

> C23 makes various standard library functions, that return a pointer
> into an input array, into macros that return a pointer to const when
> the relevant argument passed to the macro is a pointer to const.

Shouldn't this be disabled when compiling with -std=gnu17?

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Andreas Schwab, SUSE Labs, schwab@suse.de
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