target-arm: A64: provide skeleton for a64 insn decoding Provide a skeleton for a64 instruction decoding in translate-a64.c, by dividing instructions into the classes defined by the ARM Architecture Reference Manual(DDI0487A_a) section C3. Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org> Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Richard Henderson <rth@twiddle.net>