perf python: Add macro to simplify maintainance of the constants array

Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/tip-ffuchgsbr5mqu91xl9oggfss@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>