Helpers to write more compact simd code

Implemented so far

• [x] cold_for_target_arch mark a function cold for certain arches only
• [ ] cold_for_target_feature

Example

toml [dependencies] simd_helpers = "0.1" ``` rust use simdhelpers::coldfortargetarch;

// On arm and power it is the main, impl for x86_64 there is a asm-optimized variant

[coldfortargetarch("x8664")]

fn fallbacksimpleimpl() { ... } ```