Update the musl submodule to c47ad25ea3 ("iconv: harden UTF-8 output
code path against input decoder bugs").

It would be nice to reuse some of the macro structure for internal
functions, like `rem_pio2`. To facilitate this, add a `public` field and
make it available in the macro's API.

These are never available in musl, so introduce easier ways to skip them
rather than needing to exclude f16/f128 functions in three different
places.

Edition 2024 requires that we avoid this. There is a lot of code that
will need to be adjusted, so start the process here with a warning that
will show up in CI.

These appeared in a later nightly. In compiler-builtins we can apply the
suggestion, but in `libm` we need to ignore them since `fx::from_bits`
is not `const` at the MSRV.

`clippy::uninlined_format_args` also seems to have gotten stricter, so
fix those here.

This will not build correctly, the move is done as a separate step from
the rest of refactoring so git's history is cleaner.

Move implementations to `generic/` like the other functions. This also
allows us to combine the `fma` and `fma_wide` modules.

Rustc now mangles these symbols on its own, so `no_mangle` is rejected
as an error.

This is a known problem in the MinGW fmaf implementation, identified at
[1].  Make sure our implementation passes this edge case.

[1]: rust-lang/rust#140515

This module is used for both i686 and x86-64.

Get performance closer to the glibc implementations by adding assembly
fma routines, with runtime feature detection so they are used even if
not compiled with `+fma` (as the distributed standard library is often
not). Glibc uses ifuncs, this implementation stores a function pointer
in an atomic.

Results of CPU flags are also cached in order to avoid repeating the
startup time in calls to different functions. The feature detection code
is a slightly simplified version of `std-detect`.

Musl sources were used as a reference [1].

Fixes: rust-lang/rust#140452 once synced

[1]: https://github.com/bminor/musl/blob/c47ad25ea3b484e10326f933e927c0bc8cded3da/src/math/x32/fma.c