perf scripting python: Fix tuple_set_u64()

tuple_set_u64() produces a signed value instead of an unsigned value.
That works for database export but not other cases. Rename to
tuple_set_d64() for database export and fix tuple_set_u64().

Fixes: df919b40

 ("perf scripting python: Extend interface to export data in a database-friendly way")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/r/20210525095112.1399-2-adrian.hunter@intel.com