perf test brstack: Speed up running test by using tr -s instead of xargs

The brstack test runs quite slowly in software models. Part of the reason
is "xargs -n1" is quite inefficient in replacing spaces with newlines.

While that's not noticeable on normal machines, it is on software models.

Use "tr -s ' ' '\n'" instead which can do the same transformation, but is
much faster. For comparison on an M1 Macbook Pro:

  $ time seq -s ' ' 10000 | xargs -n1 > /dev/null

  real    0m2.729s
  user    0m2.009s
  sys     0m0.914s
  $ time seq -s ' ' 10000 | tr -s ' ' '\n' | grep '.' > /dev/null

  real    0m0.002s
  user    0m0.001s
  sys     0m0.001s

The "grep '.'" is also needed to remove any remaining blank lines.

Signed-off-by: James Clark <james.clark@arm.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: James Clark <james.clark@linaro.org>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241213231312.2640687-2-robh@kernel.org


Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
[robh: Drop changing loop iterations on arm64. Squash blank line fix and redo commit msg]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>