perf test: Fix perf stat --bpf-counters on hybrid machines

perf test: Fix perf stat --bpf-counters on hybrid machines

The test constantly fails on my Intel hybrid machine.  The issue was it
has two events in the output even if I only gave it one event.

  $ perf stat -e instructions -- perf test -w sqrtloop

   Performance counter stats for 'perf test -w sqrtloop':

         910,856,421      cpu_atom/instructions/                (28.05%)
      14,852,865,997      cpu_core/instructions/                (96.79%)

         1.014313341 seconds time elapsed

         1.004114000 seconds user
         0.008174000 seconds sys

Let's modify the awk script to add the values for each line and print
the total.  The variable 'i' has a number of input lines that have valid
output and variable 'c' has the sum of actual counter values.  That way
it should work on any platforms.

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>