perf cs-etm: Properly set the value of 'old' and 'head' in snapshot mode

This patch adds the necessary intelligence to properly compute the value
of 'old' and 'head' when operating in snapshot mode.  That way we can
get the latest information in the AUX buffer and be compatible with the
generic AUX ring buffer mechanic.

Tester notes:

> Leo, have you had the chance to test/review this one? Suzuki?

Sure.  I applied this patch on the perf/core branch (with latest
commit 3e4fbf36c1e3 'perf augmented_raw_syscalls: Move reading
filename to the loop') and passed testing with below steps:

  # perf record -e cs_etm/@tmc_etr0/ -S -m,64 --per-thread ./sort &
  [1] 19097
  Bubble sorting array of 30000 elements

  # kill -USR2 19097
  # kill -USR2 19097
  # kill -USR2 19097
  [ perf record: Woken up 4 times to write data ]
  [ perf record: Captured and wrote 0.753 MB perf.data ]

Signed-off-by: Mathieu Poirier <[email protected]>
Tested-by: Leo Yan <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Suzuki Poulouse <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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