perf jevents: Drop or simplify small integer values

Prior to this patch '0' would be dropped as the config values default
to 0. Some json values are hex and the string '0' wouldn't match '0x0'
as zero. Add a more robust is_zero test to drop these event terms.

When encoding numbers as hex, if the number is between 0 and 9
inclusive then don't add a 0x prefix.

Update test expectations for these changes.

On x86 this reduces the event/metric C string by 58,411 bytes.

Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Cc: Edward Baker <edward.baker@intel.com>
Cc: Perry Taylor <perry.taylor@intel.com>
Cc: Weilin Wang <weilin.wang@intel.com>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Jing Zhang <renyu.zj@linux.alibaba.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Veronika Molnarova <vmolnaro@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240131201429.792138-1-irogers@google.com