[lld] [lld,wasm] Introduce Ctx::arg (PR #119829)

[Sam Clegg via llvm-commits]

sbc100 wrote:

Can convention we have been using for PR titles is `[lld][WebAssembly]` and we use `[WebAssembly]` when its not part of the linker.   I know the whole `wasm` vs `Wasm` vs `WebAssembly` thing is inconsistent in llvm but we are fairly consistent with the PR titles.

https://github.com/llvm/llvm-project/pull/119829