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/isa/mod.rs

@@ -84,7 +84,7 @@ pub mod fde;
 mod arm32;
 
 #[cfg(feature = "arm64")]
-mod arm64;
+mod aarch64;
 
 mod call_conv;
 mod constraints;
@@ -93,6 +93,9 @@ mod encoding;
 pub mod registers;
 mod stack;
 
+#[cfg(test)]
+mod test_utils;
+
 /// Returns a builder that can create a corresponding `TargetIsa`
 /// or `Err(LookupError::SupportDisabled)` if not enabled.
 macro_rules! isa_builder {
@@ -117,7 +120,7 @@ pub fn lookup(triple: Triple) -> Result<Builder, LookupError> {
             isa_builder!(x86, "x86", triple)
         }
         Architecture::Arm { .. } => isa_builder!(arm32, "arm32", triple),
-        Architecture::Aarch64 { .. } => isa_builder!(arm64, "arm64", triple),
+        Architecture::Aarch64 { .. } => isa_builder!(aarch64, "arm64", triple),
         _ => Err(LookupError::Unsupported),
     }
 }