ART: dex2oat flag for HGraphVisualizer dump file

This patch adds a new '--dump-cfg=<file>' flag to dex2oat which
specifies the file that HGraphVisualizer will store its output into.
Until now the graph was dumped to 'art.cfg' in the current working
directory. To make Checker work with run-test, the output directory
needs to be customizable.

Change-Id: I4a940f7708b88deea5a0e51d13aed13e52199349