[lld][WebAssembly] Add `--extra-features` flag to add addional features

This flag acts just like the existing `--features` flag but instead
of replacing the set of inferred features it adds to it.

This is useful for example if you want to `--export` a mutable global
but none of the input of object were built with mutable global support.
In that case you can do `--extra-features=mutable-globals` to avoid the
linker error that would otherwise be generated in this case:

wasm-ld: error: mutable global exported but 'mutable-globals' feature not present in inputs: `__stack_pointer`. Use --no-check-features to suppress.

Differential Revision: https://reviews.llvm.org/D135831