Fail to compile in riscv because of cpufeatures unconditional dependency

[come-maiz]

I'm trying to build the lighthouse ethereum client in a risc-v unmatched board, and it fails with:

Compiling cpufeatures v0.2.12
error: This crate works only on `aarch64`, `loongarch64`, `x86`, and `x86-64` targets.
   --> /home/come-maiz/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cpufeatures-0.2.12/src/lib.rs:152:1
    |
152 | compile_error!("This crate works only on `aarch64`, `loongarch64`, `x86`, and `x86-64` targets.");
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I requested cpufeatures to support riscv, and they suggested to open an issue here to make it's dependency conditional on the architecture:

RustCrypto/utils#1087 (comment)

[michaelsproul]

Are you already acquainted with @haurog ? Maybe you could work together:

• Enable RISC-V build #8

[haurog]

I pinged @come-maiz on the Ethereum-on-arm discord. Working together to get clients run on RISC-V is exciting.

[come-maiz]

nice, let's be friends @haurog :D

[come-maiz]

Thank you @michaelsproul