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 <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/symbol.c

···
 static bool symbol__is_idle(const char *name)
 {
 	const char * const idle_symbols[] = {
+		"arch_cpu_idle",
 		"cpu_idle",
 		"cpu_startup_entry",
 		"intel_idle",