[kernel] Cabs2cil: fixes the alignement computation on packed directives.

- uses the alignment of a field type instead of its size to compute
  the field alignement.
- removes incorrect special case for flexible array members, introduced to
  circumvent a crash when computing their size. As we now use the alignment
  instead of the size, the crash no longer occurs.
- merges the implementation of two similar cases (whether there are aligned
  field attributes or not).