fix(memory): fix memory alignment for dynamic allocation

all pointers returned by arena_alloc residing in arena block should be
properly aligned

to meet neovim's alignment requirements but keeping it simple settle on
ARENA_ALIGN = MAX(sizeof(void *), sizeof(double)).

src/nvim/memory.h

@@ -45,7 +45,7 @@ typedef struct consumed_blk {
   struct consumed_blk *prev;
 } *ArenaMem;
 
-#define ARENA_ALIGN sizeof(void *)
+#define ARENA_ALIGN MAX(sizeof(void *), sizeof(double))
 
 typedef struct {
   char *cur_blk;