Simplify alloc by assuming MALLOC_IS_LISP_ALIGNED

Simplify alloc by assuming MALLOC_IS_LISP_ALIGNED

Problem reported by Hong Xu <https://bugs.gnu.org/75551#14>.
* src/alloc.c (MALLOC_IS_LISP_ALIGNED): static_assert it,
since it is true on all current Emacs platforms.
All uses simplified to assume it is true.
(xmalloc, xzalloc, xrealloc, lisp_malloc): Just use
malloc/calloc/realloc.  Since we are using the malloc-gnu
and realloc-posix modules, we need not worry about whether
these functions return a null pointer for zero-size requests.
(xrealloc): Stop worrying about no-longer-existing platforms
where realloc (nullptr, ...) did not work.
(laligned, lmalloc, lrealloc): Remove.  All uses removed.

(cherry picked from commit d3a2ec5210746a942263d5e18ee3b8190b9698e1)