Add support for 32bit systems by falling back to std::unordered_map

[juntyr]

Compiling SZ3 on 32bit systems (wasm32 in my use case) fails because of the ska_hash implementation, which expects that size_t is 64b wide. While there have been attempts to port ska_hash to 32 bits, e.g. in skarupke/flat_hash_map#18, pulling in those changes would place the maintenance burden on SZ3.

In this PR, I'm instead suggesting a simpler solution by falling back to the std::unordered_map implementation on non-64bit systems, where ska::unordered_map is not available. This immediately unlocks 32bit support with a minimal code change.

[ayzk]

Thanks for this PR.