x86/suspend: Simplify system table handling on resume

x86/suspend: Simplify system table handling on resume

load_TR() is used exclusively in the resume path, but jumps through a lot of
unnecessary hoops.  As suspend/resume is strictly on CPU0 in idle context, the
correct GDT to use is boot_gdt, which means it doesn't need saving on suspend.

Although doing more than strictly necessary, reuse load_system_tables(), which
is already used by APs on the S3 resume path.

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