target-i386: Check CR4[DE] for processing DR4/DR5

Introduce helper_get_dr so that we don't have to put CR4[DE]
into the scarce HFLAGS resource.  At the same time, rename
helper_movl_drN_T0 to helper_set_dr and set the helper flags.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
