Automatically convert returned pointers to unsigned in CAN_ADDRESS_GB mode.

[sbc100]

When we have a signature for native function and we know it returns a pointer we automatically interpret it as unsigned when it arrives in JS.

[sbc100]

Even though there were only two places where I was actually able to remove the explicit >>>= 0 in the source code there are other places where this was missing (and could leave to bugs in CAN_ADDRESS_2GB. For example here:

emscripten/src/library.js

Line 3550 in cd3596b

var ptr = _emscripten_builtin_memalign({{{ WASM_PAGE_SIZE }}}, size);