Address review comments:

- Undo temporary changes to default features (`all-arch`) and a
  signal-handler test.
- Remove `SIGTRAP` handler: no longer needed now that we've found an
  "undefined opcode" option on ARM64.
- Rename pp.rs to pretty_print.rs in machinst/.
- Only use empty stack-probe on non-x86. As per a comment in
  rust-lang/compiler-builtins [1], LLVM only supports stack probes on
  x86 and x86-64. Thus, on any other CPU architecture, we cannot refer
  to `__rust_probestack`, because it does not exist.
- Rename arm64 to aarch64.
- Use `target` directive in vcode filetests.
- Run the flags verifier, but without encinfo, when using new backends.
- Clean up warning overrides.
- Fix up use of casts: use u32::from(x) and siblings when possible,
  u32::try_from(x).unwrap() when not, to avoid silent truncation.
- Take immutable `Function` borrows as input; we don't actually
  mutate the input IR.
- Lots of other miscellaneous cleanups.

[1] cae3e6ea23/src/probestack.rs (L39)

cranelift/codegen/src/machinst/adapter.rs

@@ -4,9 +4,12 @@ use crate::binemit;
 use crate::ir;
 use crate::isa::{EncInfo, Encoding, Encodings, Legalize, RegClass, RegInfo, TargetIsa};
 use crate::machinst::*;
-use crate::regalloc::{RegDiversions, RegisterSet};
+use crate::regalloc::RegisterSet;
 use crate::settings::Flags;
 
+#[cfg(feature = "testing_hooks")]
+use crate::regalloc::RegDiversions;
+
 use std::borrow::Cow;
 use std::fmt;
 use target_lexicon::Triple;
@@ -30,7 +33,11 @@ impl TargetIsaAdapter {
 
 impl fmt::Display for TargetIsaAdapter {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "MachBackend")
+        f.debug_struct("MachBackend")
+            .field("name", &self.backend.name())
+            .field("triple", &self.backend.triple())
+            .field("flags", &format!("{}", self.backend.flags()))
+            .finish()
     }
 }