Neon intrinsic code generator

A small tool that allows to quickly generate intrinsics for the NEON architecture.

The specification for the intrinsics can be found in neon.spec.

To run and re-generate the code run the following from the root of the stdarch crate.

OUT_DIR=`pwd`/crates/core_arch cargo run -p stdarch-gen -- crates/stdarch-gen/neon.spec