Clean syscall handling

* Implements a svc handler suitable to supply as a handler for
  thread_svc_handler.
* Removes hardcoded call to tee_svc_sycall in
  thread_svc_handler.
* Removes duplicated code for unwinding of stack after
  tee_svc_enter_user_mode() replacing it with a single
  tee_svc_unwind_enter_user_mode()

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (QEMU virt and FVP)
Reviewed-by: Etienne Carriere <etienne.carriere@st.com>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org> (STM platform)