Fix build for aarch64

[seijikun]

I tried using ffsvm on aarch64, but after fixing simd_aligned, the build fails in ffsvm.
That's because in comparison to is_x86_feature_detected, the is_aarch64_feature_detected macro is in the std::arch namespace instead of std (?!).
This fixes the build for me.

[ralfbiedert]

Thanks!

[ralfbiedert]

I fixed the x86 warning caused by the unused std::arch, please let me know if broke aarch64 again, as I don't have the toolchain set up right now.

[seijikun]

ffsvm-0.9.2 builds on aarch64 now, thank you!
However, the change in ralfbiedert/simd_aligned#2 is also required for it to work.

[ralfbiedert]

Ah sorry, missed that PR. Should be published now.