More MinGW C99 printf compliance

Passing "--std=gun99" is unfortunately not sufficient to make current
MinGW compilers conform with respect to checking printf format strings.
(The C runtime seems not to have problems.)

Fix by using a "gnu_printf" format specifier not "printf".

src/target/arm11.h

@@ -33,8 +33,9 @@
 	type * variable = calloc(1, sizeof(type) * items)
 
 /* For MinGW use 'I' prefix to print size_t (instead of 'z') */
+/* Except if __USE_MINGW_ANSI_STDIO is defined with MinGW    */
 
-#ifndef __MSVCRT__
+#if (!defined(__MSVCRT__) || defined(__USE_MINGW_ANSI_STDIO))
 #define ZU		"%zu"
 #else
 #define ZU		"%Iu"