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/runtime/src/traphandlers.rs

@@ -31,7 +31,6 @@ cfg_if::cfg_if! {
         static mut PREV_SIGBUS: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();
         static mut PREV_SIGILL: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();
         static mut PREV_SIGFPE: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();
-        static mut PREV_SIGTRAP: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();
 
         unsafe fn platform_init() {
             let register = |slot: &mut MaybeUninit<libc::sigaction>, signal: i32| {
@@ -71,9 +70,6 @@ cfg_if::cfg_if! {
                 register(&mut PREV_SIGFPE, libc::SIGFPE);
             }
 
-            // on ARM64, we use `brk` to report traps, which generates SIGTRAP.
-            register(&mut PREV_SIGTRAP, libc::SIGTRAP);
-
             // On ARM, handle Unaligned Accesses.
             // On Darwin, guard page accesses are raised as SIGBUS.
             if cfg!(target_arch = "arm") || cfg!(target_os = "macos") {
@@ -91,7 +87,6 @@ cfg_if::cfg_if! {
                 libc::SIGBUS => &PREV_SIGBUS,
                 libc::SIGFPE => &PREV_SIGFPE,
                 libc::SIGILL => &PREV_SIGILL,
-                libc::SIGTRAP => &PREV_SIGTRAP,
                 _ => panic!("unknown signal: {}", signum),
             };
             let handled = tls::with(|info| {