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/isa/x64/lower.rs

@@ -14,11 +14,10 @@ use crate::isa::x64::inst::args::*;
 use crate::isa::x64::inst::*;
 use crate::isa::{x64::settings as x64_settings, x64::X64Backend, CallConv};
 use crate::machinst::lower::*;
-use crate::machinst::*;
 use crate::result::CodegenResult;
 use crate::settings::{Flags, TlsModel};
+use crate::{machinst::*, trace};
 use alloc::boxed::Box;
-use log::trace;
 use smallvec::SmallVec;
 use std::convert::TryFrom;
 use target_lexicon::Triple;