Update documentation/build_system.md

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
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.

Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org> (STM platform)
Signed-off-by: Cedric Chaumont <cedric.chaumont@st.com>