include/qemu/osdep.h: Add macros for pointer alignment

These macros provide a convenient way to n-byte align pointers up and
down and check if a pointer is n-byte aligned.

Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
Message-Id: <1461341333-19646-3-git-send-email-sergey.fedorov@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
