[Zucchini] Remove misaligned memory access from casted pointers: Part 2/2.

This CL finishes fixing aligned memory access failure brought on by:

  *reinterpret_cast<T*>(*uint8_ptr)

Part 1 (crrev.com/c/4567104) has treated direct memory access via helper
functions (e.g., GetValue<T>(), PutValue<T>()) by replacing the above
with memcpy().

Part 2 (this CL) treats indirect memory access via helper functions
GetPointer<T>() and GetArray<T>(). Guided by mvanbem@'s suggestion,
instead of eradicating reinterpret_cast<T*> use (and making massive
changes to callers), we start by simply adding the compile-time check

  static_assert(alignof(T) == 1)

to GetValue<T>() and PutValue<T>(). The prevalent use of
"#pragma align(push, 1)" for executable file format structs ensures
alignof(T) == 1 for these structs, i.e., the compiler assumes member
member variables access may be unaligned, and therefore won't emit
alignment-sensitive instructions that will trigger SIGBUS at runtime.

The check fails if T were multi-byte POD types (e.g., uint32_t).
Previously this happens in two cases, which we resolved in this CL:
1. BufferSourceTest.Get{Pointer,Array}Integral: T = uint32_t, and the
   check fail. The solution is to wrap uint32_t into new struct
   UnalignedUint32T inside a "#pragma align(push, 1)" block, then use
   the struct (and access the internal |value|).
2. dex::CodeItemParser::GetNext() calling GetArray<uint16_t>(): The
   intent of this call is to skip |code_item->insns_size| 2-byte units,
   and if the skip runs out of data, return error. Recently in
   crrev.com/c/4568608 we've updated Skip() to return false on "out of
   data". The solution is to use Skip() instead. A (passing) usage
   of GetArray<dex::TryItem>() is similarly replaced.

Fixing the above eliminates multi-byte POD usage for GetPointer<T>() and
GetArray<T>(), so for now we don't need to worry about them. If the need
arises, we can add some "Unaligned" wrapper template in the future.

Also, add BufferSourceTest.Get{Pointer,Array}IntegralMisaligned to
exercise misaligned (re. 4-byte boundary) access succeeding by using a
UnalignedUint32T wrapper.

Bug: 1446782
Change-Id: Id19a8b52eac4f43f446a9a59934f5d32f32ad6d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4583255
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1152557}

• components/zucchini/buffer_source.h[diff]
• components/zucchini/buffer_source_unittest.cc[diff]
• components/zucchini/disassembler_dex.cc[diff]