[C++] Rare bad buffer content alignment if sizeof(T) != alignof(T) (#7520)

* [C++] Add a failing unit test for #7516 (Rare bad buffer content alignment if sizeof(T) != alignof(T))

* [C++] Fix final buffer alignment when using an array of structs
* A struct can have an arbitrary size and therefore sizeof(struct) == alignof(struct)
  does not hold anymore as for value primitives.
* This patch fixes this by introducing alignment parameters to various
  CreateVector*/StartVector calls.
* Closes #7516

tests/alignment_test.fbs

@@ -0,0 +1,24 @@
+// sizeof(BadAlignmentSmall) == 12
+// alignof(BadAlignmentSmall) == 4
+struct BadAlignmentSmall {
+  var_0: uint;
+  var_1: uint;
+  var_2: uint;
+}
+
+// sizeof(BadAlignmentLarge) == 8
+// alignof(BadAlignmentLarge) == 8
+struct BadAlignmentLarge {
+  var_0: ulong;
+}
+
+table OuterLarge {
+  large: BadAlignmentLarge;
+}
+
+table BadAlignmentRoot {
+  large: OuterLarge;
+  small: [BadAlignmentSmall];
+}
+
+root_type BadAlignmentRoot;