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

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).