Use panic! instead of println! + loop in double fault handler (#687)

This avoids an endless loop when a double fault occurs while running `cargo xtest`.

src/interrupts.rs

@@ -33,8 +33,7 @@ extern "x86-interrupt" fn double_fault_handler(
     stack_frame: &mut InterruptStackFrame,
     _error_code: u64,
 ) {
-    println!("EXCEPTION: DOUBLE FAULT\n{:#?}", stack_frame);
-    loop {}
+    panic!("EXCEPTION: DOUBLE FAULT\n{:#?}", stack_frame);
 }
 
 #[cfg(test)]