[C++] Align serialized scalars by wire size

[carrerasdarren-cell]

Fixes #9182.

FlatBuffers aligns serialized scalars to their size, but several templated
builder paths used the host compiler's ABI alignment instead. On targets such
as m68k, where uint32_t is 4 bytes with 2-byte ABI alignment, this can make
PushElement insert padding between a string's length prefix and payload.
Scalar and offset vectors have the same wire-alignment mismatch.

This change:

• uses the scalar width when pre-aligning scalar length prefixes;
• uses wire width for scalar, Offset, and Offset64 vector elements while
  retaining AlignOf<T>() for generated structs;
• applies the same rule to uninitialized scalar vectors; and
• adds architecture-independent regressions with deliberately under-aligned
  32-bit and 64-bit scalar types.

Validation:

• CMake flattests: all tests passed.
• Exact statically linked m68k reproducer under qemu-m68k: stock master
  inserts two zero bytes and fails; patched code emits the canonical bytes and
  passes.