Memory allocator alignment fixes

These changes remove some assumptions about heap allocator
alignment that may not be true on unusual platforms.
* src/alloc.c (POWER_OF_2): New macro.
(ROUNDUP): Use it.
(BLOCK_ALIGN): Verify that it is a power of 2.
(aligned_alloc): Check that alignment passed to posix_memalign
satisfies POSIX restrictions.
(lisp_align_malloc): Check that size passed to aligned_alloc
satisfies C11 restrictions.
(MALLOC_IS_GC_ALIGNED): Check that GCALIGNMENT is 8, since the
code has not been verified to work with other GCALIGNMENT values
and the ice is thin here.  On GNU/Linux, malloc can return a value
that is a multiple of 8 but not 16, even though __alignof__
(max_align_t) is 16.  See:
https://gcc.gnu.org/ml/gcc-patches/2016-08/msg01902.html
(lmalloc) [USE_ALIGNED_ALLOC]: Use aligned_alloc only if size
is a multiple of alignment, since C11 says the behavior is
undefined otherwise.
(lmalloc, lrealloc): Don't use INT_ADD_WRAPV on size_t, as in
general this macro is restricted to signed types.  Remove
assertion that the result is a multiple of GCALIGNMENT, as that
need not be true.

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