[child] convert Neighbor to const void* for the address comparison (#12116)

When using some special configurations, the ot-cli-ftd will crash. The
crash path is `MessageFramer::PrepareMacHeaders()` ->
`Get<NeighborTable>().FindNeighbor()` ->
`Get<ChildTable>().Contains()`. The crash happens in the
`ChildTable::Contains()`. Here is the system crash message: `kernel:
traps: ot-cli-ftd[122376] trap invalid opcode ip:5640b7713b8e
sp:7ffd6425c5f0 error:0 in ot-cli-ftd[313b8e,5640b7400000+426000]`.

The root cause of the crash is that the CandidateParent is a 4 bytes
aligned class and the Child is a 8 bytes aligned class. When
converting the CandidateParent to Neighbor and then converting the
Neighbor to Child, the program will crash due to the alignment issues.

This commit replace the static_cast with the reinterpret_cast in
ChildTable::Contains() to convert a Neighbor to a Child.

src/core/thread/child_table.hpp

@@ -308,7 +308,7 @@ public:
      */
     bool Contains(const Neighbor &aNeighbor) const
     {
-        const Child *child = static_cast<const Child *>(&aNeighbor);
+        const void *child = &aNeighbor;
 
         return (mChildren <= child) && (child < GetArrayEnd(mChildren));
     }