[llvm] [WebAssembly] Add WebAssembly::Specifier (PR #133116)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Since wasm doesn't need a MCTargetExpr yet, I think we do not need the extra .cpp file yet.
I've moved the relocation specifier constants to `WebAssembly::S_` (The `Expr` suffix used by other targets doesn't seem useful).

https://github.com/llvm/llvm-project/pull/133116