perf_counter tools: Introduce alias member in event_symbol

By introducing alias member in event_symbol :

1. duplicate lines are removed, like:
   cpu-cycles and cycles
   branch-instructions and branches
   context-switches and cs
   cpu-migrations and migrations

2. We can also add alias for another events.

Now ./perf list looks like :

List of pre-defined events (to be used in -e):

  cpu-cycles OR cycles                     [Hardware event]
  instructions                             [Hardware event]
  cache-references                         [Hardware event]
  cache-misses                             [Hardware event]
  branch-instructions OR branches          [Hardware event]
  branch-misses                            [Hardware event]
  bus-cycles                               [Hardware event]

  cpu-clock                                [Software event]
  task-clock                               [Software event]
  page-faults                              [Software event]
  faults                                   [Software event]
  minor-faults                             [Software event]
  major-faults                             [Software event]
  context-switches OR cs                   [Software event]
  cpu-migrations OR migrations             [Software event]

  rNNN                                     [raw hardware event descriptor]

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <1245669268.17153.8.camel@localhost.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>