iosbuild.sh: specify optimization flags

explicitly set '-O3 -DNDEBUG'. setting CFLAGS on the command line
overrides the default, resulting in -O0.

Change-Id: I213979f646b1444b1d8e0eb0bb58e9b2c3cc4dd3
update ChangeLog

Change-Id: I3b930aa6cb72d17f41e52d645de1d9b2f3a0238b