[musl] adding C23 support

["Jₑₙₛ Gustedt" <jens.gustedt@inria.fr>]

Hello,
as you might have heard, C23 has been finally approved by the C
committee in January 2024 and is now in the loop with ISO to be
published mid-2024. There have been no surprises since the last time
we talked, so all patches that I already proposed some time ago should
still be valid. They can still be found at

      https://forge.icube.unistra.fr/icps/musl/-/branches

The latest is branch c23-v9 based on today's release of musl, but this
will hopefully be updated when we go along. The patches should be
stacked from the least debated to some which perhaps will never make
it into musl. So if they could be considered bottom-up for inclusion,
that would be great.

As far as I can see my patches cover most what is needed for C23
library support, with one noticeable exception which are functions for
<math.h>. These are really too much for me and my level of
expertise. But I have seen that Paul Zimmermann has some of them (the
π functions) in his project, so this could maybe be covered from
there.

Compilers are now basically there, gcc-14 and clang-18 have most what
is needed for C23 from POV of the language.

Thanks

Jₑₙₛ

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