Fix QEMU test flakiness on ARM64 macOS

On ARM64 macOS, qemu-system-x86_64 uses TCG (software emulation)
instead of KVM. The isa-debug-exit shutdown request is processed
asynchronously, so the guest vCPU can continue executing after the
first port write. This lets the fallthrough `exit_qemu(Failed)` in
`_start` overwrite the success exit code before QEMU actually shuts
down, causing intermittent test failures with exit code 35 instead
of 33.

Add `loop {}` after each `exit_qemu(Success)` in test runners so
the vCPU spins harmlessly until the shutdown completes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phil-opp

example-kernels/runner-doctest/src/lib.rs

@@ -16,6 +16,8 @@ extern crate rlibc;
 /// extern "C" fn start() {
 ///     assert_eq!(add(1, 2), 3);
 ///     unsafe { exit_qemu(ExitCode::Success); }
+///     // Spin until QEMU shuts down to avoid overwriting the exit code (TCG race on ARM64).
+///     loop {}
 /// }
 /// ```
 pub fn add(a: u32, b: u32) -> u32 {
@@ -32,6 +34,8 @@ pub fn add(a: u32, b: u32) -> u32 {
 /// extern "C" fn start() {
 ///     assert_eq!(mul(2, 3), 6);
 ///     unsafe { exit_qemu(ExitCode::Success); }
+///     // Spin until QEMU shuts down to avoid overwriting the exit code (TCG race on ARM64).
+///     loop {}
 /// }
 /// ```
 pub fn mul(a: u32, b: u32) -> u32 {
@@ -47,6 +51,9 @@ fn test_runner(tests: &[&dyn Fn()]) {
     unsafe {
         exit_qemu(ExitCode::Success);
     }
+    // Keep spinning so that the fallthrough `exit_qemu(Failed)` in `_start`
+    // cannot overwrite the exit code before QEMU shuts down (TCG race on ARM64).
+    loop {}
 }
 
 pub enum ExitCode {

example-kernels/runner-fail-reboot/src/lib.rs

@@ -14,6 +14,9 @@ pub fn test_runner(tests: &[&dyn Fn()]) {
     unsafe {
         exit_qemu(ExitCode::Success);
     }
+    // Keep spinning so that the fallthrough `exit_qemu(Failed)` in `_start`
+    // cannot overwrite the exit code before QEMU shuts down (TCG race on ARM64).
+    loop {}
 }
 
 #[test_case]

example-kernels/runner-test/src/lib.rs

@@ -13,6 +13,9 @@ pub fn test_runner(tests: &[&dyn Fn()]) {
     }
 
     unsafe { exit_qemu(ExitCode::Success); }
+    // Keep spinning so that the fallthrough `exit_qemu(Failed)` in `_start`
+    // cannot overwrite the exit code before QEMU shuts down (TCG race on ARM64).
+    loop {}
 }
 
 #[test_case]