x86: adjust xsave structure attributes

x86: adjust xsave structure attributes

The packed attribute was pointlessly used here - there are no
misaligned fields, and hence even if the attribute took effect, it
would at best lead to the compiler generating worse code.

At the same time specify the required alignment of the fpu_sse sub-
structure, such that the various typeof() uses on that field obtain
pointers to properly aligned memory (knowledge which a compiler may
want to make use of).

Also add suitable build-time checks.

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