Use _aligned_malloc on Windows.

On 32-bit Windows, malloc doesn't give us memory with sufficiently
strong alignment for AVX vectors.

Note that Windows also has a family of allocators that provide guarantee
exact misalignment.  Using these would simplify the ivar layout logic
slightly, because we wouldn't need to account for the refcount pointer.
It's not clear that it's actually worth doing though, because that logic
already exists.

Author: davidchisnall

gc_none.c

@@ -8,13 +8,26 @@
 
 static id allocate_class(Class cls, size_t extraBytes)
 {
-	intptr_t *addr = calloc(cls->instance_size + extraBytes + sizeof(intptr_t), 1);
+	size_t size = cls->instance_size + extraBytes + sizeof(intptr_t);
+	intptr_t *addr =
+#ifdef _WIN32
+	// Malloc on Windows doesn't guarantee 32-byte alignment, but we
+	// require this for any class that may contain vectors
+		_aligned_malloc(size, 32);
+	memset(addr, 0, size);
+#else
+		calloc(size, 1);
+#endif
 	return (id)(addr + 1);
 }
 
 static void free_object(id obj)
 {
+#ifdef _WIN32
+	_aligned_free((void*)(((intptr_t*)obj) - 1));
+#else
 	free((void*)(((intptr_t*)obj) - 1));
+#endif
 }
 
 static void *alloc(size_t size)