Use intptr_t in C when talking about a pointer.

This fixes a bug in TS where we'd return an int64_t when we intended
to return a pointer, confusing JavaScript as to why it has a bigint
instead of a number.

This may make it harder to support 32-bit Java platforms in the
future if we have a reason to do so, but that seems unlikely and
its possible we can simply redefine the type.