Implement C23 memset_explicit (bug 32378)

[Joseph Myers]

On Tue, 30 Sep 2025, Collin Funk wrote:

> Ah, okay. Thanks for the explanation. I am okay with __USE_MISC now, to
> avoid errors in projects where people may use different GCC versions.
> 
> I assume Joseph's V2 patch is good, but I will review it later today.

I'm currently testing a v3 (v2 missed updates to bits/string_fortified.h 
to use __USE_MISC there, and the _GNU_SOURCE conditionals in tst-fortify.c 
are no longer needed).

-- 
Joseph S. Myers
josmyers@redhat.com