Fix the configure.py TOML field for a couple LLVM options
Fix the configure.py TOML field for a couple LLVM options

The actual fields in `config.toml.example` have dashes, not underscores.

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