Add API function to restore flags outline.

Both drreg and drbbdup require restoring flag registers outline, as shown below:

   reg_t val
   uint sahf;
   reg_t newval = mcontext->xflags;
   val = (reg_t)drbbdup_get_tls_raw_slot_val(DRBBDUP_FLAG_REG_SLOT);
   sahf = (val & 0xff00) >> 8;
   newval &= ~(EFLAGS_ARITH);
   newval |= sahf;
   if (TEST(1, val)) /* seto */
       newval |= EFLAGS_OF; 
   mcontext->xflags = newval;

This is a feature request to create an API function to conveniently perform this restoration.