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_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 */