Merge pull request #12696 from 0xc0170/fix_boot_iar_vector

rtos: declare vector table type as in Technical Note 11578

Author: 0xc0170

rtos/source/TARGET_CORTEX/TOOLCHAIN_IAR/mbed_boot_iar.c

@@ -28,7 +28,7 @@
 #pragma section="CSTACK"
 #pragma section="HEAP"
 
-extern void *__vector_table;
+extern void *__vector_table[];
 extern int  __low_level_init(void);
 extern void __iar_data_init3(void);
 extern __weak void __iar_init_core(void);