perf python: Port syscall-counts to perf module

Port tools/perf/scripts/python/syscall-counts.py to a standalone script
in tools/perf/python/ using the perf module. Avoiding the embedded
interpreter and per-event dictionary allocation overhead improves
execution speed by ~4x:

```
$ perf record -e raw_syscalls:sys_enter -a sleep 1
...
$ time perf script tools/perf/scripts/python/syscall-counts.py perf
...
real    0m3.887s
user    0m3.578s
sys     0m0.308s
$ time python3 tools/perf/python/syscall-counts.py perf
...
real    0m0.953s
user    0m0.905s
sys     0m0.048s
```

Additional improvements compared to the legacy script:
- Resolve syscall names using perf.syscall_name(id, session.e_machine)
  instead of host python-audit / Util.py tables, enabling accurate
  cross-architecture perf.data analysis without external dependencies.
- Support both raw_syscalls:sys_enter (sample.id) and individual
  syscalls:sys_enter_* tracepoints (sample.__syscall_nr / sample.nr),
  filtering out invalid/corrupt (> 0xffff or negative) syscall numbers.
- Add argparse CLI options (-i/--input and optional comm filter).

Add a shell test (test_syscall_counts_python.sh) to verify the
standalone script. The legacy script and its bin wrapper are retained
temporarily during the transition to maintain bisectability and are
removed once all scripts are migrated.

Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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