[Fix]:fix at_server.c link failed when using ac6

use __ARMCC_VERSION to replace __CC_ARM

components/net/at/src/at_server.c

@@ -538,7 +538,7 @@ int at_server_init(void)
     }
 
     /* initialize the AT commands table.*/
-#if defined(__CC_ARM)                                 /* ARM C Compiler */
+#if defined(__ARMCC_VERSION)                                 /* ARM C Compiler */
     extern const int RtAtCmdTab$$Base;
     extern const int RtAtCmdTab$$Limit;
     cmd_table = (at_cmd_t)&RtAtCmdTab$$Base;