Fix GC bugs --with-wide-int and Qnil == 0
Use the same alignment for the !USE_LSB_TAG case as for the more-typical USE_LSB_TAG case. The attempt to support arbitrary alignments with !USE_LSB_TAG had subtle bugs in garbage collection once we changed the representation of symbols so that Qnil == 0. Problem reported by Eli Zaretskii (Bug#20862). * src/alloc.c (XMALLOC_HEADER_ALIGNMENT) [XMALLOC_OVERRUN_CHECK]: * src/alloc.c (vector_alignment, union aligned_Lisp_Symbol) (union aligned_Lisp_Misc, maybe_lisp_pointer, pure_alloc): Use same alignment for !USE_LSB_TAG as for USE_LSB_TAG. * src/alloc.c (POINTERS_MIGHT_HIDE_IN_OBJECTS): Remove. This optimization in the !USE_LSB_TAG case is no longer valid when symbols are represented via offsets. Change the only use to assume that pointers might hide in objects. * src/lisp.h (alignas) [!USE_LSB_TAG]: Require support in this case, too. (TAG_SYMOFFSET, XSYMBOL) [!USE_LSB_TAG]: Do not shift the offset. This is OK, because the !USE_LSB_TAG case now applies only when Lisp_Object is wider than void *, so there's no longer any need to shift the offset. Not shifting the offset means that symbol representations have the same alignment as pointers, which the GC assumes.
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