xen: move wallclock functions from x86 to common

xen: move wallclock functions from x86 to common

Remove dummy arm implementation of wallclock_time.
Use shared_info() in common code rather than x86-ism to access it, when
possible.

Define the static variable wc_sec, and the local variable sec in
update_domain_wallclock_time, as uint64_t instead of unsigned long, to
avoid size issue on arm.
Take a uint64_t sec parameter in do_settime for the same reason.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
CC: JBeulich@suse.com
CC: andrew.cooper3@citrix.com
[ ijc -- typoes in commit message ]