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.
Backports commit d0052339236072bbf08c1d600c0906126b1ab258 from qemu
In order to do this, stop using the cpu_in*/out* helpers, and instead
access address_space_io directly.
cpu_in* and cpu_out* remain for usage in the monitor, in qtest, and
in Xen.
Backports commit 3f7d84648607cc0fcb3812bb4b88978e2a7aa24f from qemu
helper_sysenter in qemu/target-i386/seg_helper.c didn't check properly if a call interrupt callback was registred.
It has been fixed by copying the helper_syscall behavior.
