feature rdseed fails to build on x86

Building feature rdseed or --all-features fails on 32-bit x86:
```
error[E0432]: unresolved import `core::arch::x86::_rdseed64_step`
--> src/entropy.rs:58:6
|
58 | use core::arch::x86::_rdseed64_step as rdseed;
| ^^^^^^^^^^^^^^^^^--------------^^^^^^^^^^
| | |
| | help: a similar name exists in the module: `_rdseed16_step`
| no `_rdseed64_step` in `core_arch::arch::x86`
```

The error occurs in src/entropy.rs line 58, where `_rdseed64_step` is imported for x86. Check the Rust core::arch documentation for x86 intrinsics to see which rdseed step functions are available on 32-bit vs 64-bit targets. Likely need to conditionally compile using `cfg(target_arch = "x86_64")` or use `_rdseed32_step` for x86. Run `cargo check --target i686-unknown-linux-gnu --features rdseed` to verify the fix.

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