Make dcache multi-target-safe

By inspection, I noticed that this code in dcache.c is not
multi-target-aware:

  /* If this is a different inferior from what we've recorded,
     flush the cache.  */

  if (inferior_ptid != dcache->ptid)

This doesn't take into account that threads of different targets may
have the same ptid.

Fixed by also storing/comparing the process_stratum_target.

Tested on x86-64-linux-gnu, native and gdbserver.

Change-Id: I4d9d74052c696b72d28cb1c77b697b911725c8d3