perf python: Fix breakage introduced by the test_attr infrastructure

perf python: Fix breakage introduced by the test_attr infrastructure

The test_attr infrastructure hooks on the sys_perf_event_open call,
checking if a variable is set and if so calling a function to intercept
calls and do the checking.

But both the variable and the function aren't on objects that are
linked on the python binding, breaking it:

  # perf test -v 15
  15: Try 'use perf' in python, checking link problems       :
  --- start ---
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  ImportError: /home/acme/git/build/perf//python/perf.so: undefined symbol: test_attr__enabled
  ---- end ----
  Try 'use perf' in python, checking link problems: FAILED!
  #

Fix it by moving the variable to one of the linked object files and
providing a stub for the function in the python.o object, that is only
linked in the python binding.

Now 'perf test' is happy again:

  # perf test 15
  15: Try 'use perf' in python, checking link problems       : Ok
  #

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-0rsca2kn44b38rgdpr3tz6n5@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>