Function add_configuration 

pub(crate) fn add_configuration(
    cfg: &mut Cfg,
    target_config: &TargetConfig,
    target: &Target,
    is_nightly_build: bool,
    is_crt_static: bool,
)

Adds target_feature = "..." cfgs for a variety of platform specific features (SSE, NEON etc.).

This is performed by checking whether a set of permitted features is available on the target machine, by querying the TargetConfig from the codegen backend.