hook up tracing to cg_cranelift

[jyn514]

this was easier than expected. here is an example of using RUSTC_LOG with a build of cranelift from rust-lang/rust:

$ RUSTC_LOG=rustc_codegen_cranelift cargo +stage1 b
   Compiling example v0.1.0 (/home/jyn/src/example)
 INFO rustc_codegen_cranelift codegen crate example
 INFO rustc_codegen_cranelift codegen crate example
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.44s

[bjorn3]

Thanks!