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GH-40674: [GLib] Don't assume gint64 and int64_t use the same type (#40736)

### Rationale for this change

GLib doesn't guarantee that `gint64` and `int64_t` use the same type:

https://docs.gtk.org/glib/types.html#gint64

> Note that on platforms with more than one 64-bit standard integer
> type, gint64 and int64_t are not necessarily implemented by the same
> 64-bit integer type. For example, on a platform where both long and
> long long are 64-bit, it might be the case that one of those types is
> used for gint64 and the other is used for int64_t.

### What changes are included in this PR?

Add explicit casts.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #40674

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