os_test: C++: disable profiling (-pg)

os_test includes C++ exception tests which are incompatible with ftrace
by design [1]. This commit makes sure the C++ tests are not compiled
with the profiling flag (-pg), which is the flag that one would use to
enable function tracing in TAs.

Link: [1] https://github.com/OP-TEE/optee_os/issues/4022


Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>