Raise a compile warning if you use an x86 intrinsic that's not supported by the target_feature

[newpavlov]

Yeah, the current handling of target features is disappointingly "not Rusty" (i.e. instead of proper compile-time checks we have unsafe everywhere). Long time ago I wrote the following proposal, but unfortunately amount of development activity in this area is very small:

[Pre-Pre-RFC] Target restriction contexts

The following text is braindump-like description of how I would like to see Rust work with various target options. (target_arch, target_feature, target_os and others) This approach aims to provide stronger compile time guarantees compared to current cfg setups, remove most of the unsafe in the stdsimd crate and integrate target parameters closer with Cargo.toml. Also it could be useful for improving Rust portability story. While RFC 2045 is an important step towards stabilization of SIMD, I don…