To support the WebAssembly<->JS barrier, return values also have to be
passed in memory. i64 return values are used by syscall/js, so must be
supported across this ABI barrier.
JavaScript does not support i64 directly, so make sure we pass a pointer
instead which can be read from JavaScript.
This is a temporary workaround which should be removed once JavaScript
supports some form of i64 (probably in the form of BigInt).
