perf counters: restructure the API

Impact: clean up new API

Thorough cleanup of the new perf counters API, we now get clean separation
of the various concepts:

 - introduce perf_counter_hw_event to separate out the event source details

 - move special type flags into separate attributes: PERF_COUNT_NMI,
   PERF_COUNT_RAW

 - extend the type to u64 and reserve it fully to the architecture in the
   raw type case.

And make use of all these changes in the core and x86 perfcounters code.

Also change the syscall signature to:

  asmlinkage int sys_perf_counter_open(

	struct perf_counter_hw_event	*hw_event_uptr		__user,
	pid_t				pid,
	int				cpu,
	int				group_fd);

( Note that group_fd is unused for now - it's reserved for the counter
  groups abstraction. )

Signed-off-by: Ingo Molnar <mingo@elte.hu>