Adds the dexfuzz fuzzing tool to ART.

This tool can mutate DEX files, and execute them with multiple backends
of ART, making sure that all backends agree on the output of the
program. This should help identify bugs in particular backends, should
they produce output that disagrees with the other backends.

Build with mmm tools/dexfuzz from within art/.

See the README for details about how to use dexfuzz.

Change-Id: I3fc5d84a08b7d142a1fdc3d43a0324ce16b43608