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/ir/function.rs

@@ -3,8 +3,6 @@
 //! The `Function` struct defined in this module owns all of its basic blocks and
 //! instructions.
 
-#![allow(unused_imports)]
-
 use crate::binemit::CodeOffset;
 use crate::entity::{PrimaryMap, SecondaryMap};
 use crate::ir;
@@ -19,7 +17,6 @@ use crate::isa::{CallConv, EncInfo, Encoding, Legalize, TargetIsa};
 use crate::regalloc::{EntryRegDiversions, RegDiversions};
 use crate::value_label::ValueLabelsRanges;
 use crate::write::write_function;
-use alloc::boxed::Box;
 use core::fmt;
 
 /// A function.