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/compile.rs

@@ -4,6 +4,7 @@ use crate::ir::Function;
 use crate::isa::TargetIsa;
 use crate::machinst::*;
 use crate::timing;
+use crate::trace;
 
 use regalloc2::RegallocOptions;
 use regalloc2::{self, MachineEnv};
@@ -27,7 +28,7 @@ pub fn compile<B: LowerBackend + TargetIsa>(
         lower.lower(b)?
     };
 
-    log::trace!("vcode from lowering: \n{:?}", vcode);
+    trace!("vcode from lowering: \n{:?}", vcode);
 
     // Perform register allocation.
     let regalloc_result = {