perf symbols: Add 'arch_cpu_idle' to the list of kernel idle symbols

When testing 'perf top' on a armhf system (32-bit, Orange Pi Zero), I
noticed that 'arch_cpu_idle' dominated, add it to the list of idle
symbols, so that we can see what is that being done when not idle.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-4q2b5g4p2hrstrhp9t2mrlho@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>