target: arc: rewrite command 'arc add-reg' as COMMAND_HANDLER

While there, fix some coding style error and remove the now unused
function jim_arc_read_reg_name_field() and the macro
JIM_CHECK_RETVAL().

Change-Id: I140b4b929978b2936f2310e0b7d1735ba726c517
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7426
Tested-by: jenkins
Reviewed-by: Evgeniy Didin <didin@synopsys.com>

src/target/arc.h

@@ -253,16 +253,6 @@ struct arc_common {
 		}				\
 	} while (0)
 
-#define JIM_CHECK_RETVAL(action)		\
-	do {					\
-		int __retval = (action);	\
-		if (__retval != JIM_OK) {	\
-			LOG_DEBUG("error while calling \"%s\"",	\
-				# action);     \
-			return __retval;	\
-		}				\
-	} while (0)
-
 static inline struct arc_common *target_to_arc(struct target *target)
 {
 	return target->arch_info;