Allow disabling wasm-opt while keeping clang optimizations

[anlavandier]

The webassembly optimizer wasm-opt doesn't support every proposal that clang/lld supports. This can lead to case where an option that is valid and recognized by emcc fails at the optimization stage because wasm-opt doesn't know of/support a specific feature.
For example, the custom page size proposal is support by LLVM since at least LLVM 21.0 but isn't supported by wasm-opt as of right now; see WebAssembly/binaryen#7694.
Allowing the wasm-opt step to be skipped would allow users to still get the clang optimizations in such cases and leave them the possibility of further optimizing the resulting wasm using either a newer version of wasm-opt or the competing optimizer from wasm-tools