So that when thread__delete() runs it can be called and free stuff tools
stashed into thread->priv, like 'perf trace' does and will use this
new facility to plug some leaks.

Added an assert(thread__priv_destructor == NULL) as suggested in Ian's
review.
Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/lkml/CAP-5=fV3Er=Ek8=iE=bSGbEBmM56_PJffMWot1g_5Bh8B5hO7A@mail.gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>