webp: build with -O2
webp: build with -O2

set LOCAL_ARM_MODE=arm as well.
on arm devices -O2 will improve performance ~11-22%. arm mode gives the
same relative improvement with -O2, but the absolute values improve
another 11-17%.

Change-Id: If735f0f85cba94639bd8f914ba02df2186fd18d3

temasek/android_external_webp,temasek/android_external_webp