lib: No need to make lpc176x SystemInit() externally visible

Now that the lpc176x code uses the armcm_boot mechanism, it is no
longer necessary for SystemInit to be externally visible.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

lib/lpc176x/device/system_LPC17xx.c

@@ -511,7 +511,7 @@ uint32_t SystemCoreClock = __CORE_CLK;/*!< System Clock Frequency (Core Clock)*/
  * @brief  Setup the microcontroller system.
  *         Initialize the System.
  */
-void __attribute__((externally_visible)) SystemInit (void)
+void SystemInit (void)
 {
 #if (CLOCK_SETUP)                       /* Clock Setup                        */
   LPC_SC->SCS       = SCS_Val;