Remove some wrong 8-byte alignment assumptions

Remove some wrong 8-byte alignment assumptions

Do not assume that 8-byte alignment suffices for all C objects,
as some platforms require 16-byte alignment for some objects,
and this will start to bite us as time goes on (e.g., if an
Emacs module ever uses an object containing a long
double, which requires 16-byte alignment on x86-64).
Conversely, on !USE_LSB_TAG platforms, do not insist on
aligning Lisp objects to a multiple of 8, as this is not
needed for high-order tag bits.
* src/alloc.c (LISP_ALIGNMENT, MALLOC_IS_LISP_ALIGNED):
New constants.
(XMALLOC_BASE_ALIGNMENT, XMALLOC_HEADER_ALIGNMENT):
Removed.  All uses replaced by LISP_ALIGNMENT.
(aligned_alloc, laligned, lmalloc, lrealloc, union aligned_Lisp_Misc)
(maybe_lisp_pointer, pure_alloc):
Use LISP_ALIGNMENT rather than GCALIGNMENT.
(aligned_alloc): Do not worry about an alignment of
LISP_ALIGNMENT when MALLOC_IS_LISP_ALIGNED, as the code never
uses aligned_alloc with alignment == LISP_ALIGNMENT in that case.
(__alignof__): Remove.  All uses removed.
(MALLOC_IS_GC_ALIGNED): Remove.
All uses replaced with MALLOC_IS_LISP_ALIGNED.
(vector_alignment): Remove.
All uses replaced with LISP_ALIGNMENT.
* src/alloc.c (mark_maybe_pointer):
* src/emacs-module.c (value_to_lisp_bits):
Do not assume GCALIGNMENT == 1 << GCTYPEBITS, as GCALIGNMENT
is 1 on !USE_LSB_TAG platforms now.
* src/lisp.h (GCALIGNMENT) [!USE_LSB_TAG]: Now 1.
(struct Lisp_Symbol, union vectorlike_header, struct Lisp_Cons)
(struct Lisp_String): Simplify test for verifying alignment.