perf tools: Resolve idle thread cmdline for perf trace

The cmd-trace tool used the cmdline file and resolved the idle
thread using a hardcoded check for the 0 task pid.

Now we have a centralized way to do that from perf using
register_idle_thread() API.

Before:
	:0-0     [000]     0.000000: irq_handler_entry: irq=0 handler=name
	:0-0     [000]     0.000000: irq_handler_entry: irq=0 handler=name

After:
	[idle]-0     [000]     0.000000: irq_handler_entry: irq=0 handler=name
	[idle]-0     [000]     0.000000: irq_handler_entry: irq=0 handler=name

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <1251693921-6579-2-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>