tests/libc_newlib: fix pointers comparison for llvm

With llvm and samr21-xpro, I could not directly do 'printf == iprintf'.
But doing `(printf - iprintf) == 0` correctly checked if they are equal.

Co-authored-by: Kaspar Schleiser <kaspar@schleiser.de>

tests/libc_newlib/main.c

@@ -58,15 +58,18 @@ static void test_newlib(void)
     /*
      * Be sure `iprintf` and `printf` are used when `newlib` is included as
      * they should be visible in the final elf file for compile time tests
+     *
+     * With llvm and samr21-xpro, I could not directly do 'printf == iprintf'.
+     * But doing `(printf - iprintf) == 0` correctly checked if they are equal.
      */
 
 #ifdef MODULE_NEWLIB
 #ifdef MODULE_NEWLIB_NANO
     /* Nano maps iprintf to printf */
-    TEST_ASSERT(iprintf == printf);
+    TEST_ASSERT_MESSAGE((printf - iprintf) == 0, "iprintf == printf");
 #else
     /* Normal newlib does not */
-    TEST_ASSERT(iprintf != printf);
+    TEST_ASSERT_MESSAGE((printf - iprintf) != 0, "iprintf != printf");
 #endif
 #endif
 }