target-i386: fix segment flags for SMM and VM86 mode

With the next patch, these need to be correct or VM86 tasks
have the wrong CPL.  The flags are basically what the Intel VMX
documentation say is mandatory for entry into a VM86 guest.

For consistency, SMM ought to have the same flags except with
CPL=0.

Tested-by: Kevin O'Connor <kevin@koconnor.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>