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/s390x/inst/emit.rs

@@ -7,6 +7,7 @@ use crate::isa::s390x::inst::*;
 use crate::isa::s390x::settings as s390x_settings;
 use crate::machinst::reg::count_operands;
 use crate::machinst::{Reg, RegClass};
+use crate::trace;
 use core::convert::TryFrom;
 use regalloc2::Allocation;
 
@@ -3239,7 +3240,7 @@ impl MachInstEmit for Inst {
             }
 
             &Inst::VirtualSPOffsetAdj { offset } => {
-                log::trace!(
+                trace!(
                     "virtual sp offset adjusted by {} -> {}",
                     offset,
                     state.virtual_sp_offset + offset

cranelift/codegen/src/isa/s390x/mod.rs

@@ -72,7 +72,6 @@ impl TargetIsa for S390xBackend {
         let flags = self.flags();
         let (vcode, regalloc_result) = self.compile_vcode(func, flags.clone())?;
 
-        let want_disasm = want_disasm || log::log_enabled!(log::Level::Debug);
         let emit_result = vcode.emit(&regalloc_result, want_disasm, flags.machine_code_cfg_info());
         let frame_size = emit_result.frame_size;
         let value_labels_ranges = emit_result.value_labels_ranges;