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)

crates/jit/src/link.rs

@@ -142,12 +142,17 @@ cfg_if::cfg_if! {
             pub fn ___chkstk();
         }
         const PROBESTACK: unsafe extern "C" fn() = ___chkstk;
+    } else if #[cfg(not(any(target_arch = "x86_64", target_arch = "x86")))] {
+        // As per
+        // https://github.com/rust-lang/compiler-builtins/blob/cae3e6ea23739166504f9f9fb50ec070097979d4/src/probestack.rs#L39,
+        // LLVM only has stack-probe support on x86-64 and x86. Thus, on any other CPU
+        // architecture, we simply use an empty stack-probe function.
+        extern "C" fn empty_probestack() {}
+        const PROBESTACK: unsafe extern "C" fn() = empty_probestack;
     } else {
         extern "C" {
             pub fn __rust_probestack();
         }
-        static PROBESTACK: unsafe extern "C" fn() = empty_probestack;
+        static PROBESTACK: unsafe extern "C" fn() = __rust_probestack;
     }
 }
-
-extern "C" fn empty_probestack() {}