use alignas

lib/nimalloc.h

@@ -1,4 +1,7 @@
+#include <stdalign.h>
 
 // this type is used in alloc.nim. It is intended to ensure 16 byte
 // alignments for all allocations.
-typedef struct {} __attribute__ ((aligned (16))) NimAlign16Type;
+typedef struct {
+  alignas(16) char unused;
+} NimAlign16Type;