Refactor the atom_js2c target to include javascript from multiple dirs.

Refactor the atom_js2c target to include javascript from multiple dirs.

Before invoking js2c, copy all files that must be embedded into the shared
intermediate directory, and modify the js2c wrapper script to include all files
from that directory(which is passed as argument).

This allows the build system to embed files that don't share a common base
directory, such as javascript generated at build time.