target/arm: Use unallocated_encoding for aarch32

target/arm: Use unallocated_encoding for aarch32

Promote this function from aarch64 to fully general use.
Use it to unify the code sequences for generating illegal
opcode exceptions.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190807045335.1361-11-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>