Reorder pg_sequence columns to avoid alignment issue

On AIX, doubles are aligned at 4 bytes, but int64 is aligned at 8 bytes.
Our code assumes that doubles have alignment that can also be applied to
int64, but that fails in this case.  One effect is that
heap_form_tuple() writes tuples in a different layout than
Form_pg_sequence expects.

Rather than rewrite the whole alignment code, work around the issue by
reordering the columns in pg_sequence so that the first int64 column
naturally comes out at an 8-byte boundary.