perf build: fix "argument list too long" in second location

perf build: fix "argument list too long" in second location

Turns out that displaying "RM $^" via quiet_cmd_rm can also upset the
shell and cause it to display "argument list too long".

Trying to quote $^ doesn't help.

In the end, *not* displaying the (potentially long) list of files is
probably the right thing to do for a "quiet" message, anyway. Instead,
let's display a count of how many files were removed. There is always
V=1 if more detail is required.

  TEST    linux/tools/perf/pmu-events/metric_test.log
  RM      ...634 orphan file(s)...
  LD      linux/tools/perf/util/perf-util-in.o

Also move the comment regarding xargs before the rule, so it doesn't
show up in the build output.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>