Implement C23 memset_explicit (bug 32378)

[Paul Eggert]

On 2025-09-30 08:33, Joseph Myers wrote:
> If the user is using GCC 15 or later (or any compiler defaulting to C23 or
> a later language version), C23 functions*will* be visible by default

Yes, good point, so the question here isn't about GCC 15; it's about 
older GCCs and non-GCCs that do not claim conformance to C23. That's an 
important set of use cases even today, and making memset_explicit 
visible would be useful for these cases.