hhvm: tell start-stop-daemon to chdir to $HHVM_RUN_DIR

jemalloc tries to write heap profiles to the current working directory by
default, so it's good for that path to be writable, and '/' (the current
working directory) isn't. So have start-stop-daemon chdir to /run/hhvm.

Change-Id: Ibde0c31bf5af5435fafaeae6cf30a8e20edc3551