Fix GCC7 warnings about switch-case fallthroughs

GCC7 with -Wextra warns about switch-case blocks which fallthrough with
"this statement may fall through [-Werror=implicit-fallthrough=]". This
can be fixed by adding "special" comments: "/* fallthrough */".

See https://gcc.gnu.org/gcc-7/changes.html

Change-Id: Iba0be791dbdd86984489b2d9a0592bb59828da1e
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/4174
Tested-by: jenkins

src/target/arm_adi_v5.c

@@ -346,8 +346,10 @@ static int mem_ap_write(struct adiv5_ap *ap, const uint8_t *buffer, uint32_t siz
 			case 4:
 				outvalue |= (uint32_t)*buffer++ << 8 * (address++ & 3);
 				outvalue |= (uint32_t)*buffer++ << 8 * (address++ & 3);
+				/* fallthrough */
 			case 2:
 				outvalue |= (uint32_t)*buffer++ << 8 * (address++ & 3);
+				/* fallthrough */
 			case 1:
 				outvalue |= (uint32_t)*buffer++ << 8 * (address++ & 3);
 			}
@@ -509,8 +511,10 @@ static int mem_ap_read(struct adiv5_ap *ap, uint8_t *buffer, uint32_t size, uint
 			case 4:
 				*buffer++ = *read_ptr >> 8 * (3 - (address++ & 3));
 				*buffer++ = *read_ptr >> 8 * (3 - (address++ & 3));
+				/* fallthrough */
 			case 2:
 				*buffer++ = *read_ptr >> 8 * (3 - (address++ & 3));
+				/* fallthrough */
 			case 1:
 				*buffer++ = *read_ptr >> 8 * (3 - (address++ & 3));
 			}
@@ -519,8 +523,10 @@ static int mem_ap_read(struct adiv5_ap *ap, uint8_t *buffer, uint32_t size, uint
 			case 4:
 				*buffer++ = *read_ptr >> 8 * (address++ & 3);
 				*buffer++ = *read_ptr >> 8 * (address++ & 3);
+				/* fallthrough */
 			case 2:
 				*buffer++ = *read_ptr >> 8 * (address++ & 3);
+				/* fallthrough */
 			case 1:
 				*buffer++ = *read_ptr >> 8 * (address++ & 3);
 			}

src/target/arm_disassembler.c

@@ -3299,6 +3299,7 @@ static int t2ev_data_immed(uint32_t opcode, uint32_t address,
 		case 0x10:
 		case 0x12:
 			is_signed = true;
+			/* fallthrough */
 		case 0x18:
 		case 0x1a:
 			/* signed/unsigned saturated add */

src/target/target.c

@@ -3684,7 +3684,7 @@ COMMAND_HANDLER(handle_bp_command)
 				addr = 0;
 				return handle_bp_command_set(CMD_CTX, addr, asid, length, hw);
 			}
-
+			/* fallthrough */
 		case 4:
 			hw = BKPT_HARD;
 			COMMAND_PARSE_ADDRESS(CMD_ARGV[0], addr);