Call SystemInit before loading the .data section for TEENSY3.1 when using GCC_ARM

This is needed because it seems the watchdog otherwise resets the board
before SystemInit is reached even though I'm not sure why the watchdog
timer expires so early.

libraries/mbed/targets/cmsis/TARGET_Freescale/TARGET_K20XX/TARGET_TEENSY3_1/TOOLCHAIN_GCC_ARM/startup_MK20DX256.s

@@ -192,6 +192,13 @@ __isr_vector:
     .globl    Reset_Handler
     .type    Reset_Handler, %function
 Reset_Handler:
+/*
+ *    Call SystemInit before loading the .data section to prevent the watchdog
+ *    from resetting the board.
+ */
+    ldr    r0, =SystemInit
+    blx    r0
+
 /*     Loop to copy data from read only memory to RAM. The ranges
  *      of copy from/to are specified by following symbols evaluated in
  *      linker script.
@@ -212,8 +219,6 @@ Reset_Handler:
 
 .Lflash_to_ram_loop_end:
 
-    ldr    r0, =SystemInit
-    blx    r0
     ldr    r0, =_start
     bx    r0
     .pool

libraries/mbed/targets/cmsis/TARGET_Freescale/TARGET_K20XX/TARGET_TEENSY3_1/system_MK20DX256.c

@@ -100,6 +100,8 @@ uint32_t SystemCoreClock = DEFAULT_SYSTEM_CLOCK;
    -- SystemInit()
    ---------------------------------------------------------------------------- */
 void SystemInit (void) {
+  /* SystemInit MUST NOT use any variables from the .data section, as this section is not loaded yet! */
+
 #if (DISABLE_WDOG)
   /* Disable the WDOG module */
   /* WDOG_UNLOCK: WDOGUNLOCK=0xC520 */