Change compiler flag to `-O2`
Change compiler flag to `-O2`

Summary:
@public

Changes compiler flag from `-O3` to `-O2`

Saves 5.3 KiB compressed / 12 KiB uncompressed on 32bit Android.

Java on-device benchmarks show a slight improvement.

Reviewed By: astreet

Differential Revision: D13916987

fbshipit-source-id: e6bc474889e24a90ec473c8ae8c5029224c97eba

facebook/css-layout,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/css-layout