1、解决未在rtconfig.h中定义“RT_USING_CPU_FFS”宏时，使用arm-gcc编译会出现__rt_ffs() 函数重定义，导致编译不通过的问题； (#5755)

2、解决bsp/rm48x50工程使用arm-gcc编译下载后，程序运行出现prefetch abort异常的问题。

libcpu/arm/cortex-r4/cpu.c

@@ -87,9 +87,11 @@ void rt_hw_cpu_dcache_disable()
 }
 
 #elif __GNUC__
+#ifdef RT_USING_CPU_FFS
 int __rt_ffs(int value)
 {
     return __builtin_ffs(value);
 }
 #endif
+#endif
 /*@}*/

libcpu/arm/cortex-r4/start_gcc.S

@@ -54,6 +54,23 @@ _reset:
 @-------------------------------------------------------------------------------
 @ Initialize CPU Registers
 @ After reset, the CPU is in the Supervisor mode (M = 10011)
+        mov r0, #0x0000
+        mov r1, #0x0000
+        mov r2, #0x0000
+        mov r3, #0x0000
+        mov r4, #0x0000
+        mov r5, #0x0000
+        mov r6, #0x0000
+        mov r7, #0x0000
+        mov r8, #0x0000
+        mov r9, #0x0000
+        mov r10, #0x0000
+        mov r11, #0x0000
+        mov r12, #0x0000
+        mov r13, #0x0000
+        mrs r1, cpsr
+        msr spsr_cxsf, r1
+
         cpsid  if, #19
 
 #if defined (__VFP_FP__) && !defined(__SOFTFP__) && defined(RT_VFP_LAZY_STACKING)