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)

tests/custom_signal_handler.rs

@@ -122,7 +122,7 @@ mod tests {
                 .downcast::<Trap>()?;
             assert!(
                 trap.message()
-                    .starts_with("wasm trap: out of bounds"),
+                    .starts_with("wasm trap: out of bounds memory access"),
                 "bad trap message: {:?}",
                 trap.message()
             );
@@ -149,7 +149,7 @@ mod tests {
                 .downcast::<Trap>()?;
             assert!(trap
                 .message()
-                .starts_with("wasm trap: out of bounds"));
+                .starts_with("wasm trap: out of bounds memory access"));
         }
         Ok(())
     }