Add a --standalone-wasm flag to wasm-emscripten-finalize

[kripken]

The flag indicates that we want to run the wasm by itself, without JS support. In that case we don't emit JS dynCalls etc., and we also emit a wasi _start if there is a main, i.e., we try to use the current conventions in the wasm-only space.

[kripken]

Ok, I renamed the flag from wasi to pure-wasm. The flag basically means we don't want to depend on JS, so no dynCalls etc., and we want to run in wasm-only environments, so we use wasi etc. conventions, which for now means using a wasi _start. I agree in the future we can look for a nicer way to set up that function in emscripten (but atm it seems to be like we'd need something even messier on that side).

[kripken]

Renamed again, to --standalone-wasm. Based on offline discussion that seems to have the most support. We may tweak the emscripten flag, but for the binaryen side I think this is good enough?