Replace "VPx" by "AVx"

aom/aom_encoder.h

aom/aomcx.h

aom_util/aom_thread.c

aom_util/aom_thread.h

av1/av1_dx_iface.c

av1/common/onyxc_int.h

av1/common/thread_common.c

av1/common/thread_common.h

av1/decoder/decodeframe.c

av1/decoder/decoder.c

av1/decoder/decoder.h

av1/decoder/dthread.c

av1/decoder/dthread.h

av1/encoder/encoder.c

av1/encoder/encoder.h

av1/encoder/ethread.c

build/make/iosbuild.sh

examples/aom_temporal_svc_encoder.c

examples/simple_decoder.c

libs.mk