hwc: use TARGET_SCREEN_WIDTH/TARGET_SCREEN_HEIGHT to set initial overlay size

Change-Id: I8f976b8104b9d4a95d2e8edb4f6a87e459bd10c6
hwc: allow building in Thumb mode on ARM

On an ARMv7 CPU like the OMAP3, Thumb mode brings a significant
reduction in code size without performance penalty (for that matter, the
reduced code size will reduce icache footprint, probably increasing
performance slightly).

Change-Id: I54ba1361a41f2df7ec3ae9ef7529638a2a18d25d