[clang] Add flag to opt out of wasm-opt (PR #95208)

[Alex Crichton via cfe-commits]

alexcrichton wrote:

I've opened https://github.com/llvm/llvm-project/pull/98373 to avoid running `wasm-opt` for the `wasm32-wasip2` target by default. It also fixes a minor issue with this PR where if `wasm-opt` were disabled it would skip running the linker entirely.

https://github.com/llvm/llvm-project/pull/95208