mempool: make alignment/rounding 64-bit compatible

Minimum alignment and rounding must be done on a word boundary. Let's
replace _ALIGN4() with WB_UP() which is equivalent on 32-bit targets,
and 64-bit aware.

Also enforce a minimal alignment on the memory pool. This is making
a difference mostly on64-bit targets where the widely used 4-byte
alignment is not sufficient.

The _ALIGN4() macro has no users left so it is removed.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

include/kernel.h

@@ -4159,13 +4159,13 @@ struct k_mem_pool {
  * @req K-MPOOL-001
  */
 #define K_MEM_POOL_DEFINE(name, minsz, maxsz, nmax, align)		\
-	char __aligned(align) _mpool_buf_##name[_ALIGN4(maxsz) * nmax	\
+	char __aligned(WB_UP(align)) _mpool_buf_##name[WB_UP(maxsz) * nmax \
 				  + _MPOOL_BITS_SIZE(maxsz, minsz, nmax)]; \
 	struct sys_mem_pool_lvl _mpool_lvls_##name[Z_MPOOL_LVLS(maxsz, minsz)]; \
 	Z_STRUCT_SECTION_ITERABLE(k_mem_pool, name) = { \
 		.base = {						\
 			.buf = _mpool_buf_##name,			\
-			.max_sz = _ALIGN4(maxsz),			\
+			.max_sz = WB_UP(maxsz),				\
 			.n_max = nmax,					\
 			.n_levels = Z_MPOOL_LVLS(maxsz, minsz),		\
 			.levels = _mpool_lvls_##name,			\