fix: x86_64以外のアーキテクチャ向けビルドエラーを修正

[kawarimidoll]

Apple Silicon（aarch64）のMacbookでビルドを試みたところ、アーキテクチャ起因のビルドエラーが発生したため、非x86環境でもビルドできるよう調整を行いました。

select_in_word_optimizedメソッドを導入し、x86かつ可能ならselect_in_word_pdepを、それ以外（非x86含む）ならselect_in_wordを呼び出すようにしています。

発生したエラー

❯ cargo build --release --features cli
   Compiling pico-args v0.5.0
   Compiling byteorder v1.5.0
   Compiling rustmigemo v0.1.4 (/Users/kawarimidoll/ghq/github.com/kawarimidoll/rustmigemo)
error: This macro cannot be used on the current target.
                                   You can prevent it from being used in other architectures by
                                   guarding it behind a cfg(any(target_arch = "x86", target_arch = "x86_64")).
  --> src/migemo/bit_vector.rs:83:16
   |
83 |             if is_x86_feature_detected!("bmi2") {
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this error originates in the macro `is_x86_feature_detected` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0432]: unresolved import `std::arch::x86_64`
 --> src/migemo/bit_vector.rs:1:16
  |
1 | use std::arch::x86_64::_pdep_u64;
  |                ^^^^^^ could not find `x86_64` in `arch`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `rustmigemo` (lib) due to 2 previous errors