drreg should provide a full aflags barrier including flags-in-xax

The drreg routines that get spilled values back into app registers are supposed to double as "barriers" for lazy restoration, helping to support clients that insert internal control flow. However, drreg_restore_app_aflags() only puts the flags into the eflags register and does not restore the app's xax register on x86. This issue is a feature request to provide a full barrier routine for the flags. For now a client can also call drreg_get_app_value() on xax as a workaround.