A wasm module can be either a command or a reactor, so it can export
either `_start` or `_initialize`. Currently, if a command module is run,
`iwasm` still looks for `_initialize`, resulting in the warning:
`can not find an export 0 named _initialize in the module`.
Change to look for `_initialize` only if `_start` not found to resolve the issue.
6cb2ea4935