Stop using MBEDTLS_PRINTF_SIZET

Since Mbed TLS 3.6.0, all officially supported versions of Visual Studio
a printf function family that is sufficiently compliant to C99 for our
purposes, in particular supporting `%zu` for `size_t`. The only platform
without `%zu` that we semi-officially support is older versions of MinGW,
still used in our CI. MinGW provides either a Windows legacy printf or a
standards-compliant printf depending on the value of
`__USE_MINGW_ANSI_STDIO` when compiling each C file. Force the use of the
compliant version. Don't rely on `MBEDTLS_PRINTF_SIZET`, which is defined in
`<mbedtls/debug.h>` and no longer considers the Windows legacy version in
Mbed TLS >= 4.1.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

programs/test/selftest.c

@@ -5,6 +5,15 @@
  *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
  */
 
+/* On Mingw-w64, force the use of a C99-compliant printf() and friends.
+ * This is necessary on older versions of Mingw and/or Windows runtimes
+ * where snprintf does not always zero-terminate the buffer, and does
+ * not support formats such as "%zu" for size_t and "%lld" for long long.
+ */
+#if !defined(__USE_MINGW_ANSI_STDIO)
+#define __USE_MINGW_ANSI_STDIO 1
+#endif
+
 #define MBEDTLS_DECLARE_PRIVATE_IDENTIFIERS
 
 #include "mbedtls/build_info.h"
@@ -441,8 +450,7 @@ int main(int argc, char *argv[])
             }                                                           \
         } else {                                                        \
             mbedtls_printf("Padding checks only implemented for types of size 2, 4 or 8" \
-                           " - cannot check type '" #TYPE "' of size %" MBEDTLS_PRINTF_SIZET \
-                           "\n",       \
+                           " - cannot check type '" #TYPE "' of size %zu\n", \
                            sizeof(TYPE));                                       \
             mbedtls_exit(MBEDTLS_EXIT_FAILURE);                       \
         }                                                               \