introduce TARGET_CFX_CFLAGS

Summary:
This opens up a huge door of possibilities. Have a bit on the personal queue that are all dependent on this change.
These are only set if the build variant is codefirex

Test Plan: win

Reviewers: randall.rushing, Mustaavalkosta, Roach, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D320