Don't embed the pypy build dir in the exeutable.

This bloats things too much, and it's not needed for anything but the
actual pypy interpreter.  Just manully link/append files to the generated
javascript as a separate step.
Expose the JS backend as an actual --backend=js option.

This is a pretty trivial little wrapper around the logic for the C
backend, but IMHO it is *conceptually* a separate backend.