Fix hash_instantiated_test and hash_test on big-endian

[miladfarca]

The two tests are failing on big-endian platforms after 63c77e4.

The new bit-packing code stores packed bits in the LSB of a uint64_t, then feeds raw bytes via reinterpret_cast. On big-endian, the LSB is at the end of the memory layout, so the partial-word path reads leading zero bytes instead of the actual data, causing hash collisions between distinct inputs.

Fix by byte-swapping the word to little-endian layout before feeding it to the combiner.

[miladfarca]

/cc @derekmauro

[derekmauro]

Thanks for the fix. The new preferred way to do this is something like:

if constexpr (absl::endian::native == absl::endian::big) {
  word = absl::byteswap(word);
}

[miladfarca]

Thank you for reviewing, I've updated it to use the new method.