Add support for targets with 32-bit ptrs

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Is your feature request related to a problem or challenge? Please describe what you are trying to do.
When testing Ballista on a Raspberry Pi cluster using a 32-bit OS, it was noted that whilst we don't officially support 32-bit builds in DataFusion/Ballista at present, we only actually have one specific test case that fails for that arch. To ensure DF's continued robustness across architectures, I'd like to add some minimal CI support there (e.g. get the tests running on 32-bit Linux) .

As a prerequisite for that work (see apache/datafusion#273) I'm proposing that we do precisely the same for Arrow.

Describe the solution you'd like

• Add a new CI check that runs the tests using a 32-bit Linux build
• Fix any existing arch-related assumptions in the codebase (e.g. hardcoding ptr sizes at 8 bytes instead of std::mem::size_of::<usize>())

Describe alternatives you've considered

• Explicitly disallowing compilation for 32-bit targets: this might impact on future possibilities like targeting wasm32 for Rust-in-browser use cases
• Doing nothing: this feels lazy 🤷‍♂️