wasm: add legacy target

[aykevl]

This should keep at least some compatibility with older browsers. It isn't complete though: wasi-libc is still built with the bulk-memory feature enabled. But it may be good enough for some uses.

If needed, I guess we could ship a separate wasi-libc build with the bulk-memory feature disabled.

[codefromthecrypt]

maybe in this description mention the browsers we are talking about (ex which threshold version has bulk-memory, and before that requires this build?)