perf python: Port syscall-counts-by-pid to perf module

Port tools/perf/scripts/python/syscall-counts-by-pid.py to a standalone
script in tools/perf/python/ using the perf module. Avoiding the
embedded interpreter and per-event dictionary overhead improves
execution speed by ~3.8x:

```
$ perf record -e raw_syscalls:sys_enter -a sleep 1
...
$ time perf script tools/perf/scripts/python/syscall-counts-by-pid.py perf
...
real    0m3.852s
user    0m3.512s
sys     0m0.336s
$ time python3 tools/perf/python/syscall-counts-by-pid.py perf
...
real    0m1.011s
user    0m0.963s
sys     0m0.048s
```

Additional improvements compared to the legacy script:
- Resolve architecture-specific syscall names via
  perf.syscall_name(id, session.e_machine) instead of host python-audit
  tables.
- Support both raw_syscalls:sys_enter and individual syscalls:sys_enter_*
  tracepoints, and filter out invalid (> 0xffff or negative) syscall IDs.
- Support filtering by numeric PID as well as command name (comm), and
  resolve process command names via session.find_thread(pid).

Add a shell test (test_syscall_counts_by_pid_python.sh) to verify the
standalone script.

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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