perf stat: Add user_time and system_time events

It bothered me that during benchmarking using 'perf stat' (to collect
for example CPU cache events) I could not simultaneously retrieve the
times spend in user or kernel mode in a machine readable format.

When running 'perf stat' the output for humans contains the times
reported by rusage and wait4.

  $ perf stat -e cache-misses:u -- true

   Performance counter stats for 'true':

             4,206      cache-misses:u

       0.001113619 seconds time elapsed

       0.001175000 seconds user
       0.000000000 seconds sys

But 'perf stat's machine-readable format does not provide this information.

  $ perf stat -x, -e cache-misses:u -- true
  4282,,cache-misses:u,492859,100.00,,

I found no way to retrieve this information using the available events
while using machine-readable output.

This patch adds two new tool internal events 'user_time' and
'system_time', similarly to the already present 'duration_time' event.

Both events use the already collected rusage information obtained by
wait4 and tracked in the global ru_stats.

Examples presenting cache-misses and rusage information in both human
and machine-readable form:

  $ perf stat -e duration_time,user_time,system_time,cache-misses -- grep -q -r duration_time .

   Performance counter stats for 'grep -q -r duration_time .':

        67,422,542 ns   duration_time:u
        50,517,000 ns   user_time:u
        16,839,000 ns   system_time:u
            30,937      cache-misses:u

       0.067422542 seconds time elapsed

       0.050517000 seconds user
       0.016839000 seconds sys

  $ perf stat -x, -e duration_time,user_time,system_time,cache-misses -- grep -q -r duration_time .
  72134524,ns,duration_time:u,72134524,100.00,,
  65225000,ns,user_time:u,65225000,100.00,,
  6865000,ns,system_time:u,6865000,100.00,,
  38705,,cache-misses:u,71189328,100.00,,

Signed-off-by: Florian Fischer <florian.fischer@muhq.space>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20220420102354.468173-3-florian.fischer@muhq.space
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>