Misalignment of small structs in a Vector (C++)

[bexcite]

C++, generated headers create small struct vectors with incorrect alignment. Seen in AMD/x84 and Mac M1 architectures.

I've traced the bug and it was introduced by this PR: #7520

Also, I've made a fix (#7883), but it seems lost in PRs without issues, so I'm posting the issue to give it more weight.

The issue:
Small structs that are less than uoffset_t in size without any special alignment rules may result in a broken Vector layout that reads back with different values.

Example of the flatbuffers:

// sizeof(JustSmallStruct) == 2
// alignof(JustSmallStruct) == 1
struct JustSmallStruct {
  var_0: uint8;
  var_1: uint8;
}

table SmallStructs {
  small_structs: [JustSmallStruct];
}

now if a Vector of just 3 values of JustSmallStruct: { 2, 1 }, { 3, 1 }, { 4, 1 } is created with FlatBufferBuilder::CreateVectorOfStructs<T>() we get this binary Vector that is when read back get zeros in the first JustSmallStruct item:

03 00 00 00 00 00 02 01 03 01 04 01
          ^    ^
          |    |
  vector len   |
               |
         followed by two zero bytes of an alignment
         that was added because uoffset_t required
         4 bytes alignment!?

More confusing is that FlatBufferBuilder::CreateUninitializedVectorOfStructs<T>() method doesn't have this behavior and not adding "2 bytes zeros" before writing the vector len element.

PR with a fix and tests and annotated Flatbuffers that shows the issue is here: #7883