Fix handling of systems with no Js bigint integration

When we run the node tests we use node v14 tests (since node v14 is
vendored with Emscripten). Node v14 has no Js bigint integration
unless the --experimental-wasm-bigint flag is passed. So only the
node tests really notice if we get this right. Turns out, it didn't
work. We can't call a JavaScript function with 64 bit integer arguments
without bigint integration.

In ffi_call, we are trying to call a wasm function that takes 64 bit
integer arguments. dynCall is designed to do this. We need to go back
to tracking the signature when we don't have WASM_BIGINT, and then use
dynCall. This works better now that emscripten can dynamically fill in
extra dynCall wrappers:
emscripten-core/emscripten#17328

On the other hand, for the closures we are not getting a function pointer
as a first argument. We need to make our own wasm legalizer adaptor that
splits 64 bit integer arguments and then calls the JavaScript trampoline,
then the JavaScript trampoline reassembles them, calls the closure, then
splits the result (if it's a 64 bit integer) and the adaptor puts it back
together.