Align compilation flag optee_os/optee_test(teetest)

Add plat-stm: use -mfloat-abi=soft
to fix VFP register register arguments error during testsuite linking.
It defines GCC to generate output containing library calls for floating-point operations.
optee_test(teetest) is statically linked with new client/linux driver so far.

Change-Id: I8e13beabfe3b6f7e86ad5419e25bebf3b6080629
Signed-off-by: Cedric Chaumont <cedric.chaumont@st.com>