Fix drain_reloc_list alignment bug

Fix drain_reloc_list alignment bug

* src/pdumper.c (dump_charset): Use alignof (struct charset),
not alignof (int), since struct charset might be more strictly
aligned than int.  I think this is just a minor performance
issue, but we might as well use the correct alignment.
(drain_reloc_list): Use an alignment instead of a size for the
output alignment.  This prevents undefined behavior when
alignof (struct emacs_reloc) == 8 and sizeof (dump_off) == 4
when building on x86-64 with gcc -fsanitize=undefined.