x86: clean up GDTs

- Add a 16-bit code and data segment to cstart.S.
- Move ring 3 segment descriptors at the end of the GDT, and lay them out
  as required by syscall/sysret in cstart64.S.
- Only define TSS_INTR for 32-bit, and place it where cstart64.S puts the
  32-bit ring 0 segment.
- Document that the same data segment can be used for 32 and 64-bit code.
- Mark all descriptors as accessed.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>