Simplify enforcement of object address alignment

Simplify enforcement of object address alignment

* lisp.h (struct Lisp_Symbol): Remove explicit padding.
(struct Lisp_Misc_Any): Likewise.
(struct Lisp_Free): Likewise.
* alloc.c (union aligned_Lisp_Symbol): Define.
(SYMBOL_BLOCK_SIZE, struct symbol_block): Use union
aligned_Lisp_Symbol instead of struct Lisp_Symbol.
(union aligned_Lisp_Misc): Define.
(MARKER_BLOCK_SIZE, struct marker_block): Use union
aligned_Lisp_Misc instead of union Lisp_Misc.
(Fmake_symbol, allocate_misc, gc_sweep): Adjust