Fix compatibility on unsupported CPU architectures

[liushuyu]

This pull request fixes the issue where the simd-json fallback code path isn't working for unsupported CPU architectures.

#[allow(dead_code)] is added for the handle_unicode_codepoint function since this function is not used when building the fallback logic.

An alternative method would be changing the Cargo.toml file to gate runtime-detection feature behind x86_64 and x86 architectures.

[Licenser]

I like it, thanks!

[Licenser]

released as 0.13.8 thanks a lot :)