cranelift: Introduce a feature to enable trace logs (#4484)

* Don't use `log::trace` directly but a feature-enabled `trace` macro
* Don't emit disassembly based on the log level

cranelift/codegen/src/machinst/buffer.rs

@@ -147,8 +147,8 @@ use crate::machinst::{
     BlockIndex, MachInstLabelUse, TextSectionBuilder, VCodeConstant, VCodeConstants, VCodeInst,
 };
 use crate::timing;
+use crate::trace;
 use cranelift_entity::{entity_impl, SecondaryMap};
-use log::trace;
 use smallvec::SmallVec;
 use std::convert::TryFrom;
 use std::mem;