Fix shift overflow warning for 32-bit ARM builds

[rascani]

Summary

Use if constexpr to prevent GCC from compiling the 64-bit memory offset shift on 32-bit systems where size_t is 32 bits. Also add validation that rejects models requiring >32-bit offsets on 32-bit systems.

/home/rja/executorch/runtime/executor/tensor_parser_exec_aten.cpp: In
function 'executorch::runtime::Result<void*>
executorch::runtime::deserialization::{anonymous}::getMemPlannedPtr(const
executorch_flatbuffer::AllocationDetails*, size_t,
executorch::runtime::HierarchicalAllocator*)':
/home/rja/executorch/runtime/executor/tensor_parser_exec_aten.cpp:66:62:
warning: left shift count >= width of type [-Wshift-count-overflow]
   66 |     memory_offset |= static_cast<size_t>(memory_offset_high) << 32;
      |
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

Test lan

examples/arm/setup.sh --i-agree-to-the-contained-eula
source examples/arm/arm-scratch/setup_path.sh
backends/cortex_m/test/build_test_runner.sh

[pytorch-bot]

🔗 Helpful Links

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 6fe7840 with merge base 9cbe754 ():

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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