target-arm: fix UMAAL instruction

UMAAL should use unsigned multiply instead of signed.

This patch fixes this issue by handling UMAAL separately from
UMULL/UMLAL/SMULL/SMLAL as these instructions are different
enough. It also explicitly list instructions in case and catch
nonexistent instruction as illegal. Also fixes a few style issues.

This fixes the issues reported in
https://bugs.launchpad.net/qemu/+bug/696015

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
