Fix `asan/TestCases/Darwin/scrible.cpp` to work on platforms where `long` is not 64-bits.

[delcypher]

Summary

Previously the test was failing on platforms where long was less than
64-bits wide (e.g. older WatchOS simulators and arm64_32) because the
padding field was too small.

The test currently relies on the my_object->isa being scribbled or
left unmodified after my_object is freed. However, this was not the
case because the isa pointer intersected with
ChunkHeader::free_context_id. free_context_id starts at the
beginning of user memory but it only initialized once the memory is
freed. This caused the isa pointer to change after it was freed
leading to the test crashing.

To fix this the padding field has been made explicitly 64-bits wide
(same size as ChunkHeader::free_context_id).

rdar://75806757

compiler-rt/test/asan/TestCases/Darwin/scribble.cpp

 // RUN: %clang_asan -O2 %s -o %t
 // RUN: %run %t 2>&1 | FileCheck --check-prefix=CHECK-NOSCRIBBLE %s
 // RUN: %env MallocScribble=1 MallocPreScribble=1 %run %t 2>&1 | FileCheck --check-prefix=CHECK-SCRIBBLE %s
 // RUN: %env_asan_opts=max_free_fill_size=4096 %run %t 2>&1 | FileCheck --check-prefix=CHECK-SCRIBBLE %s
 
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
 struct Isa {
   const char *class_name;
 };
 
 struct MyClass {
-  long padding;
+  // User memory and `ChunkHeader` overlap. In particular the `free_context_id`
+  // is stored at the beginning of user memory when it is freed. That part of
+  // user memory is not scribbled and is changed when the memory is freed. This
+  // test relies on `isa` being scribbled or unmodified after memory is freed.
+  // In order for this to work the start of `isa` must come after whatever is in
+  // `ChunkHeader` (currently the 64-bit `free_context_id`). The padding here is
+  // to ensure this is the case.
+  uint64_t padding;
   Isa *isa;
   long data;
 
   void print_my_class_name();
 };
 
 __attribute__((no_sanitize("address")))
 void MyClass::print_my_class_name() {