use -Os for thumb for real this time

previously i did some testing that showed using -Os thumb2 was a massive
improvement over -O3.  in the final patchset i accidentally unset TARGET_THUMB_O
completely and set TARGET_ARM_O to -Os which broke camera and later "fixes" broke
bluetooth on HH/FLO.

EPIC FACEPALM

Change-Id: I1b44dc4cff7471af2bdbc3b9d679daec38823e9b