@mcosta
You are right, QT just uses the configured compiler in kits. So, in mkspec, I added the new options,
QMAKE_CFLAGS_RELEASE = -O3 -march=armv7-a -mtune=cortex-a8 -mfpu=neon -mfloat-abi=softfp
QMAKE_CXXFLAGS_RELEASE = -O3 -march=armv7-a -mtune=cortex-a8 -mfpu=neon -mfloat-abi=softfp
QMAKE_CFLAGS_DEBUG = -O3 -march=armv7-a -mtune=cortex-a8 -mfpu=neon -mfloat-abi=softfp
QMAKE_CXXFLAGS_DEBUG = -O3 -march=armv7-a -mtune=cortex-a8 -mfpu=neon -mfloat-abi=softfp
in /usr/local/Trolltech/QtEmbedded-4.8.5-arm/mkspecs/qws/linux-arm-g++/qmake.conf.
And that works for solving my problem.
Thanks.