Make const ptr assign as function call in clang

- This can act as memory barrier in clang to avoid
  read before assign of a const ptr

Signed-off-by: LoveSy <shana@zju.edu.cn>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/const_hack.c

@@ -0,0 +1,16 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * Trick to assign a const ptr with barrier for clang
+ *
+ * Copyright (C) 2021 by YU Jincheng <shana@zju.edu.cn>
+ *
+ * Licensed under GPLv2 or later, see file LICENSE in this source tree.
+ */
+#include "libbb.h"
+
+#if defined(__clang_major__) && __clang_major__ >= 9
+void FAST_FUNC XZALLOC_CONST_PTR(const void *pptr, size_t size)
+{
+	ASSIGN_CONST_PTR(pptr, xzalloc(size));
+}
+#endif