arm32: unwind: mark tee_svc_do_call() with .cantunwind

The assembly function tee_svc_do_call() manipulates the stack pointer
but does not use the proper unwind directives when doing so. As a
result, the compiler can't generate proper unwind information. This can
lead to crashes or infinite loops if unwinding is performed at runtime.
Given that there is nothing of much interest below this function, we
simply add a .cantundwind directive to stop unwinding here.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

core/arch/arm/tee/arch_svc_a32.S

@@ -42,8 +42,8 @@
  */
 FUNC tee_svc_do_call , :
 UNWIND(	.fnstart)
+UNWIND(	.cantunwind)
 	push	{r5-r9, lr}
-UNWIND(	.save	{r5-r9, lr})
 	mov	r7, sp
 	mov	r8, r0
 	mov	r9, r1