x86/emul: Fix the decoding of segment overrides in 64bit mode

x86/emul: Fix the decoding of segment overrides in 64bit mode

Explicit segment overides other than %fs and %gs are documented as ignored by
both Intel and AMD.

In practice, this means that:

 * Explicit uses of %ss don't actually yield #SS[0] for non-canonical
   memory references.
 * Explicit uses of %{e,c,d}s don't override %rbp/%rsp-based memory references
   to yield #GP[0] for non-canonical memory references.

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