Fix 64bit long return issue

The most significant 32 bits of a 64 bit long return value get passed in the global tempRet0, but this fails when asm.js and/or closure are enabled as tempRet0 was declared again inside the asm closure. This fix exports tempRet0 via an accessor method Runtime.getTempRet0() which is visible with or without optimisations being enabled.

See https://github.com/kripken/emscripten/issues/2265