[musl] C23 support, v11

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

Hello,
there is now a v.11 of the patches at

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

The changes are that I tried to rebase such that patches that handle
things from the same standard clause / header are closer together.

One other thing that changed, and that we should perhaps discuss a
bit, is to which version number of `__STDC_VERSION__` we refer. As far
as I can see, gcc and clang have the following strategy for
`-std=c2x`:

 - as long as the compiler support is not complete, the version macro
   has a value that is bigger than `201710L` but smaller than the
   final value `202311L`.

 - when support is considered to be sufficiently complete they switch
   to `202311L`, probably gcc-14 and clang-19

Sufficiently here means that all language features that have no
specific feature tests are implemented. What I see will still be
lacking are `#embed`, `[[reproducible]]` and `[[unsequenced]]`, but
all of these have feature tests.

The first wave of the patches only uses tests for `< 202311L` that is
it supposes all or nothing. Users that use early support compilers for
C23 could fall through the cracks with these. Since both compilers
started early with their C23 support, there are quite a number of
compilers that are concerned, 4 or 5 versions of each, gcc and clang.

On top of this there is patch 592fcd9a that relaxes part of these
tests to just `> 201710L` such that users already get some C23 support
for features that had been implemented early on. Then there are also
patches 8f145eea and 1af530a1

        C23: deal with intermediate C versions for stdbool.h
        C23: deal with intermediate C versions for assert.h

That provide intermediate versions of the corresponding features as
they had been developed over the years.

Jₑₙₛ

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