introduce unaligned.h

introduce unaligned.h

Rather than open-coding commonly used constructs in yet more places when
pulling in zstd decompression support (and its xxhash prereq), pull out
the custom bits into a commonly used header (for the hypervisor build;
the tool stack and stubdom builds of libxenguest will still remain in
need of similarly taking care of). For now this is limited to x86, where
custom logic isn't needed (considering this is going to be used in init
code only, even using alternatives patching to use MOVBE doesn't seem
worthwhile).

For Arm64 with CONFIG_ACPI=y (due to efi-dom0.c's re-use of xz/crc32.c)
drop the not really necessary inclusion of xz's private.h.

No change in generated code.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>