add CMD_NAME macro for command handlers

By introducing the CMD_NAME macro, this parameter may be integrated
as args[-1] in command.[ch], without touching any other call sites.

src/target/cortex_m3.c

@@ -1902,7 +1902,7 @@ COMMAND_HANDLER(handle_cortex_m3_mask_interrupts_command)
 
 	if (target->state != TARGET_HALTED)
 	{
-		command_print(cmd_ctx, "target must be stopped for \"%s\" command", cmd);
+		command_print(cmd_ctx, "target must be stopped for \"%s\" command", CMD_NAME);
 		return ERROR_OK;
 	}