perf build: Prevent "argument list too long" error

Due to a recent change, building perf may result in a build error when
it is trying to "prune orphans".

The file list passed to "rm" may exceed what the shell can handle.

The build will then abort with an error like this:

TEST [...]/arm64/build/linux-custom/tools/perf/pmu-events/metric_test.log
make[5]: /bin/sh: Argument list too long
make[5]: *** [pmu-events/Build:217: prune_orphans] Error 127
make[5]: *** Waiting for unfinished jobs....
make[4]: *** [Makefile.perf:773: [...]/tools/perf/pmu-events/pmu-events-in.o] Error 2
make[4]: *** Waiting for unfinished jobs....
make[3]: *** [Makefile.perf:289: sub-make] Error 2

Processing the arguments via "xargs", instead of passing the list of
files directly to "rm" via the shell, prevents this issue.

Fixes: 36a1b0061a584430 ("perf build: Reduce pmu-events related copying and mkdirs")
Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>