[BSP] fix the compiling issue in mb9bf568r bsp

bsp/mb9bf568r/CMSIS/DeviceSupport/gcc/startup_mb9bf56xr.S

@@ -150,8 +150,10 @@ __cs3_interrupt_vector_cortex_m:
     .section .cs3.reset,"x",%progbits
     .thumb_func
     .globl  __cs3_reset_cortex_m
+    .globl  _start
     .type   __cs3_reset_cortex_m, %function
 __cs3_reset_cortex_m:
+_start:
     .fnstart
     LDR     R0, =SystemInit
     BLX     R0

bsp/mb9bf568r/rtconfig.py

@@ -42,7 +42,7 @@ if PLATFORM == 'gcc':
     DEVICE = ' -mcpu=cortex-m4 -mthumb'
     CFLAGS = DEVICE
     AFLAGS = ' -c' + DEVICE + ' -x assembler-with-cpp'
-    LFLAGS = DEVICE + ' -Wl,--gc-sections,-Map=rtthread-' + BOARD_NAME + '.map,-cref,-u,Reset_Handler -T rtthread-' + BOARD_NAME + '.ld'
+    LFLAGS = DEVICE + ' -Wl,--gc-sections,-Map=rtthread-' + BOARD_NAME + '.map,-cref,-u,Reset_Handler -T rtthread-' + BOARD_NAME + '.ld -nostartfiles'
 
     CPATH = ''
     LPATH = ''

bsp/mb9bf568r/rtthread-mb9bf568r.ld

@@ -18,7 +18,6 @@
 OUTPUT_FORMAT ("elf32-littlearm", "elf32-bigarm", "elf32-littlearm")
 ENTRY(_start)
 SEARCH_DIR(.)
-GROUP(-lgcc -lc -lcs3 -lcs3unhosted -lcs3micro)
 
 MEMORY
 {