Merge pull request #1876 from cpforbes/gcc9

Backport fix for building ports/unix with gcc9 from micropython

lib/utils/printf.c

@@ -103,9 +103,11 @@ STATIC void strn_print_strn(void *data, const char *str, size_t len) {
     strn_print_env->remain -= len;
 }
 
-#if defined(__GNUC__) && !defined(__clang__)
+#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ < 9
 // uClibc requires this alias to be defined, or there may be link errors
 // when linkings against it statically.
+// GCC 9 gives a warning about missing attributes so it's excluded until
+// uClibc+GCC9 support is needed.
 int __GI_vsnprintf(char *str, size_t size, const char *fmt, va_list ap) __attribute__((weak, alias ("vsnprintf")));
 #endif