Expose the features computed from LLVM in `cfg!`
Expose the features computed from LLVM in `cfg!`

Instead of relying on the features explicitly passed through the
command line, compute them from the LLVM `TargetMachine`.

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