Rollup merge of #56749 - alexcrichton:adx, r=gnzlbg
Rollup merge of #56749 - alexcrichton:adx, r=gnzlbg

x86: Add the `adx` target feature to whitelist

Requested in rust-lang-nursery/stdsimd#322 this is hopefully the first
step!

aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust