Build emulator according to the host architecture

Quoted from [bug 897727](https://bugzilla.mozilla.org/show_bug.cgi?id=897727) [comment 6](https://bugzilla.mozilla.org/show_bug.cgi?id=897727#c6)

> Change the emulator's build system to build only 32bit or only 64bit according to the host architecture.
> This works here (ubuntu 12.04 64bit) and ./run-emulator.sh just works. (Thanks to Gabriele Svelto for help)