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GH-37735: [C++][FreeBSD] Suppress a shorten-64-to-32 warning (#38004)

### Rationale for this change

It's caused by `backtrace()` and `backtrace_symbols_fd()` signatures are different on Linux and FreeBSD (`int` vs `size_t`).

Linux:

```c
extern int backtrace (void **__array, int __size) __nonnull ((1));
extern void backtrace_symbols_fd (void *const *__array, int __size, int __fd)
     __THROW __nonnull ((1));
```

FreeBSD:

```c
size_t backtrace(void **, size_t);
int backtrace_symbols_fd(void *const *, size_t, int);
```

### What changes are included in this PR?

Use `auto` to ignore the difference.

### Are these changes tested?

Yes on FreeBSD 12.

### Are there any user-facing changes?

No.
* Closes: #37735

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>