Add SIMD examples in section Processor

[meven]

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Demonstrate a very simple way to implement code to use simd in https://rust-lang-nursery.github.io/rust-cookbook/hardware/processor.html Demonstrate especially the macros and function about processor feature detection and fallback and a simple use of simd, rather that an advanced use of the feature.

#[cfg(all(any(target_arch = "x86", target_arch = "x86_64"),
      target_feature = "avx2"))]
fn foo() {
    #[cfg(target_arch = "x86")]
    use std::arch::x86::_mm256_add_epi64;
    #[cfg(target_arch = "x86_64")]
    use std::arch::x86_64::_mm256_add_epi64;

    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    {
        if is_x86_feature_detected!("avx2") {
            return unsafe { foo_avx2() };
        }
    }

References: https://github.com/rust-lang/rfcs/blob/master/text/2325-stable-simd.md https://blog.rust-lang.org/2018/06/21/Rust-1.27.html