target-arm: Fix SWI (SVC) instruction in M profile.

When do_interrupt_v7m is called with EXCP_SWI, the PC already
points to the next instruction. Don't modify it here.
Signed-off-by: NAlex Rozenman <Alex_Rozenman@mentor.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target-arm/helper.c

@@ -1736,7 +1736,7 @@ static void do_interrupt_v7m(CPUARMState *env)
         armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_USAGE);
         return;
     case EXCP_SWI:
-        env->regs[15] += 2;
+        /* The PC already points to the next instruction.  */
         armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_SVC);
         return;
     case EXCP_PREFETCH_ABORT: