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/filetests/src/test_vcode.rs

@@ -4,11 +4,9 @@ use cranelift_codegen::isa::lookup;
 use cranelift_codegen::settings;
 use cranelift_codegen::Context as CodegenContext;
 use cranelift_reader::{TestCommand, TestOption};
-use target_lexicon::Triple;
 
 use log::info;
 use std::borrow::Cow;
-use std::str::FromStr;
 use std::string::String;
 
 struct TestVCode {
@@ -41,15 +39,13 @@ impl SubTest for TestVCode {
     }
 
     fn needs_isa(&self) -> bool {
-        false
+        true
     }
 
     fn run(&self, func: Cow<Function>, context: &Context) -> SubtestResult<()> {
+        let triple = context.isa.unwrap().triple().clone();
         let func = func.into_owned();
 
-        let triple =
-            Triple::from_str(&self.arch).map_err(|_| format!("Unknown arch: '{}'", self.arch))?;
-
         let mut isa = lookup(triple)
             .map_err(|_| format!("Could not look up backend for arch '{}'", self.arch))?
             .finish(settings::Flags::new(settings::builder()));