!481 修正非主cpu的启动模式不正确的问题

Merge pull request !481 from jesven/fix_secondary_boot

libcpu/arm/cortex-a/start_gcc.S

@@ -666,12 +666,12 @@ after_enable_mmu2:
     cps #Mode_FIQ
     ldr sp, =irq_stack_2_limit
 
-    cps #Mode_SVC
-    ldr sp, =svc_stack_2_limit
-
     cps #Mode_ABT
     ldr sp, =abt_stack_2_limit
 
+    cps #Mode_SVC
+    ldr sp, =svc_stack_2_limit
+
     /* initialize the mmu table and enable mmu */
 #ifndef RT_USING_USERSPACE
     bl rt_hw_mmu_init