x86emul: {L,S}{G,I}DT ignore operand size overrides in 64-bit mode

This affects not only the layout of the data (always 2+8 bytes), but
also the contents (no truncation to 24 bits occurs).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
This will only apply cleanly on top of
https://lists.xenproject.org/archives/html/xen-devel/2016-11/msg00170.html.

--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -4424,7 +4424,7 @@ x86_emulate(
             fail_if(ops->read_segment == NULL);
             if ( (rc = ops->read_segment(seg, &sreg, ctxt)) )
                 goto done;
-            if ( op_bytes == 2 )
+            if ( !mode_64bit() && op_bytes == 2 )
                 sreg.base &= 0xffffff;
             if ( (rc = ops->write(ea.mem.seg, ea.mem.off+0,
                                   &sreg.limit, 2, ctxt)) ||
@@ -4447,7 +4447,7 @@ x86_emulate(
                                   !is_canonical_address(base), EXC_GP, 0);
             sreg.base = base;
             sreg.limit = limit;
-            if ( op_bytes == 2 )
+            if ( !mode_64bit() && op_bytes == 2 )
                 sreg.base &= 0xffffff;
             if ( (rc = ops->write_segment(seg, &sreg, ctxt)) )
                 goto done;