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GH-38090: [C++][Emscripten] orc: Suppress shorten-64-to-32 warnings (#38097)

### Rationale for this change

We need explicit cast to use `int64_t` for `size_t` on Emscripten.

### What changes are included in this PR?

Explicit casts.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #38090

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