[clang] [llvm] [WebAssembly] Add intrinsics to wasm_simd128.h for all FP16 instructions (PR #106465)

[Brendan Dahl via cfe-commits]

brendandahl wrote:

> Would it make sense to put these declarations behind `#ifdef __wasm_fp16__` so that they aren't declared if fp16 support isn't enabled?

I could do that, if that's preferred. I followed what the relaxed instructions did and use the target attribute `__target__("fp16")`. 

https://github.com/llvm/llvm-project/pull/106465