Use `_start` rather than `main` for the default entry point

[sbc100]

We already do this in STANDALONE_WASM mode. In this most we have crt1.o which defines start which calls __wasm_eval_ctors then main then exit.

Doing the same thing for the normal code path should simplify the codebase and could be a codesize win (since we would no longer need to export __wasm_eval_ctors to JS).

[sbc100]

My plan is for emscripten by default to use _emscripten_start(int argc, char* argv[]) rather than _start() which is used in standalone mode. This will mean we won't get any code size regressions due to the module having to call back to JS to get argv values.