Fix warnings exposed by GCC8

Fix warnings exposed by GCC8

gcc (GCC) 8.1.0 generates new warnings and thus fails the build.

The ARM disassembler warnings actually exposed a bug in SMALW, SMULW and
SMUL instructions decoding.

Reported by Eimers on IRC.

Change-Id: I200c70f75a9e07a1f13a592addc1c5fb37714440
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/4526
Tested-by: jenkins
Reviewed-by: Jiri Kastner <cz172638@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>