Add opt-in graphite loop transformation flags

to enable graphite for testing set ENABLE_GRAPHITE := true in your
board config.  The flags are not applied for host modules or when LOCAL_CLANG is defined.
Explicit inter-module opt-out possible by defining LOCAL_DISABLE_GRAPHITE in Android.mk

Change-Id: I669d31c380c912013d048a57bc460e01e3e7969e