Some little Cranelift logging things (#4624)

* Cranelift: Don't print "skipped TEST can't run aarch64" on x64, etc

It's way too noisy. Move it to the logs.

* Cranelift: Enable Cranelift trace logs in `clif-util` by default

* cranelift-filetest: use `log::warn!` for warnings

Instead of `println!`

* rustfmt

cranelift/filetests/src/runone.rs

@@ -41,9 +41,11 @@ pub fn run(
         Ok(testfile) => testfile,
         Err(e) => {
             if e.is_warning {
-                println!(
+                log::warn!(
                     "skipping test {:?} (line {}): {}",
-                    path, e.location.line_number, e.message
+                    path,
+                    e.location.line_number,
+                    e.message
                 );
                 return Ok(started.elapsed());
             }

cranelift/filetests/src/test_run.rs

@@ -114,7 +114,7 @@ impl SubTest for TestRun {
         let host_isa = build_host_isa(true, context.flags.clone(), vec![]);
         let requested_isa = context.isa.unwrap();
         if let Err(e) = is_isa_compatible(context, host_isa.as_ref(), requested_isa) {
-            println!("{}", e);
+            log::info!("{}", e);
             return Ok(());
         }