tcg/aarch64: implement sign/zero extend operations

implement the optional sign/zero extend operations with the dedicated
aarch64 instructions.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-id: 51AC9A58.40502@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>