x86: don't build platform hypercall helpers multiple times

x86: don't build platform hypercall helpers multiple times

... to eliminate the resulting duplicate symbols. This includes
dropping an odd per-CPU variable left from 32-bit days: Now that we
only care about 64-bit builds, converting the uint64_t needing
passing to a void pointer is no problem anymore.

Since the COMPAT handling section needs to be re-organized for this
anyway, also adjust a few other shortcomings (like declarations not
being visible at the point of the respective definition, risking both
to get out of sync).

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