[libunwind] Silence -Wunused-parameter warnings in Unwind-wasm.c (PR #125412)

[Yuriy Chernyshov via cfe-commits]

georgthegreat wrote:

@MaskRay, we build libunwind using our own build system with the clang default set of warnings enabled.

There is already a couple of `[[maybe_unused]]` attributes in this library, so I do not bring anything new:
```
(dflt) thegeorg at jakku:~/contrib/llvm/libunwind at main$ rg unused]]
src/Unwind-EHABI.cpp
890:[[gnu::unused]] static uint64_t

src/UnwindCursor.hpp
3050:  [[maybe_unused]] const int Result = syscall(
```

https://github.com/llvm/llvm-project/pull/125412