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

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

Differential Revision: https://reviews.llvm.org/D109409