TARGET_linux-arm: -Os for TARGET_thumb_CFLAGS

Improved mean quadrant score by 814 points with 20 repetitions and lowers the range by 150 compared
to using -O3.  Tested on the nexus 7 flo.

Change-Id: I3395c8764ff3851db9765dab0d46f32a998d66d5
data: http://pastebin.com/2V28x8SP
change default to linaro's 4.8

requires related manifest commit or the testing instructions there-in

Change-Id: Ie9e220e079dac04d548a004a6b0f3de4aad73417