LibC: Replace [[gnu::aligned(16)]] with C++ standard alignas(16)

Userland/Libraries/LibC/mallocdefs.h

@@ -69,7 +69,7 @@ struct ChunkedBlock : public CommonHeader {
     size_t m_next_lazy_freelist_index { 0 };
     FreelistEntry* m_freelist { nullptr };
     size_t m_free_chunks { 0 };
-    [[gnu::aligned(16)]] unsigned char m_slot[0];
+    alignas(16) unsigned char m_slot[0];
 
     void* chunk(size_t index)
     {