If user has already enabled profiling support in the kernel
(for oprofile, old-style profiling of ftrace) then offer up
perfcounters with a y default in interactive kconfig sessions.

Still keep it off by default otherwise.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>