perf thread_map: Add method to map all threads in the system

Reusing the thread_map__new_by_uid() proc scanning already in place to
return a map with all threads in the system.

Based-on-a-patch-by: Jin Yao <yao.jin@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/n/tip-khh28q0wwqbqtrk32bfe07hd@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>