target/i386: enable A20 automatically in system management mode

Ignore env->a20_mask when running in system management mode.

Reported-by: Anthony Xu <anthony.xu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1494502528-12670-1-git-send-email-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>