llvm/llvm-project#125412 removed parameter names
to suppress `-Wunused-parameter` in their own build system. As a result
now we have this error:
```console
../../../system/lib/libunwind/src/Unwind-wasm.c:105:62: error: omitting the parameter name in a function definition is a C23 extension [-Werror,-Wc23-extensions]
  105 | _LIBUNWIND_EXPORT void _Unwind_SetIP(struct _Unwind_Context *, uintptr_t) {}
      |
```

We have three ways to fix it?
1. Re-add parameter names. But this will later clash with
   llvm/llvm-project#125412.
2. Add `-std=c23` to the cflags. But this only applies to .c files and
   our `get_cflags` function in `system_libs.py` doesn't take a file
   name so we can't apply different cflags depending on the file
   extensions. While it's possible to add the filename paramater to
   `get_cflags`, it is a bigger refactoring.
3. Suppress the warning using `-Wno-c23-extension`.

This does 3, which is the quickest. But eventually we may need to change
what llvm/llvm-project#125412 did.

This updates libunwind from 20.1.8 to LLVM 21.1.8:
https://github.com/llvm/llvm-project/releases/tag/llvmorg-21.1.8

Additional change:
- Add `-Wno-c23-extensions` to libunwind's cflags:
0070206
  llvm/llvm-project#125412 removed parameter names
  to suppress `-Wunused-parameter` in their own build system, but this
  causes these errors instead for us:
  ```console
 ../../../system/lib/libunwind/src/Unwind-wasm.c:105:62: error: omitting
the parameter name in a function definition is a C23 extension
[-Werror,-Wc23-extensions]
105 | _LIBUNWIND_EXPORT void _Unwind_SetIP(struct _Unwind_Context *,
uintptr_t) {}
          |
  ```
  This suppresses the warnings.

This updates libunwind from 20.1.8 to LLVM 21.1.8:
https://github.com/llvm/llvm-project/releases/tag/llvmorg-21.1.8

Additional change:
- Add `-Wno-c23-extensions` to libunwind's cflags:
emscripten-core@0070206
  llvm/llvm-project#125412 removed parameter names
  to suppress `-Wunused-parameter` in their own build system, but this
  causes these errors instead for us:
  ```console
 ../../../system/lib/libunwind/src/Unwind-wasm.c:105:62: error: omitting
the parameter name in a function definition is a C23 extension
[-Werror,-Wc23-extensions]
105 | _LIBUNWIND_EXPORT void _Unwind_SetIP(struct _Unwind_Context *,
uintptr_t) {}
          |
  ```
  This suppresses the warnings.

Reverts #125412

See the discussion in #125412 for why this is necessary. The summary is
that:

- Eliding arguments is the C23 extension, but libunwind builds its C
files with `-std=c99`, so this change broke the build.
- `-Wno-unused-parameter` is part of the build for libunwind, so the
codebase does allow them.

….c" (#175776)

Reverts llvm/llvm-project#125412

See the discussion in #125412 for why this is necessary. The summary is
that:

- Eliding arguments is the C23 extension, but libunwind builds its C
files with `-std=c99`, so this change broke the build.
- `-Wno-unused-parameter` is part of the build for libunwind, so the
codebase does allow them.

…75776)

Reverts llvm#125412

See the discussion in llvm#125412 for why this is necessary. The summary is
that:

- Eliding arguments is the C23 extension, but libunwind builds its C
files with `-std=c99`, so this change broke the build.
- `-Wno-unused-parameter` is part of the build for libunwind, so the
codebase does allow them.