Helpers to write more compact simd code

Implemented so far

• cold_for_target_arch mark a function cold for certain arches only
• cold_for_target_feature

Example

[dependencies]
simd_helpers = "0.1"

use simd_helpers::cold_for_target_arch;

// On arm and power it is the main, impl for x86_64 there is a asm-optimized variant
#[cold_for_target_arch("x86_64")]
fn fallback_simple_impl() { ... }