xen/x86: Introduce static inline wrappers for l{idt,gdt,ldt,tr}()

xen/x86: Introduce static inline wrappers for l{idt,gdt,ldt,tr}()

This avoids indirection and parameter constraint issues.  Doing so relaxes the
load_LDT() constraints from %ax to any general purpose register.  The helpers
are upgraded to full compiler barriers, because nothing good will come of
having these reordered with respect to other segment accesses.

The triple-fault reboot method stays as is, to avoid the int3 possibly getting
moved relative to the lidt.

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