tools/nolibc: stdint: use __SIZE_TYPE__ for size_t

Otherwise both gcc and clang may generate warnings about type
mismatches:

sysroot/mips/include/string.h:12:14: warning: mismatch in argument 1 type of built-in function 'malloc'; expected 'unsigned int' [-Wbuiltin-declaration-mismatch]
   12 | static void *malloc(size_t len);
      |              ^~~~~~

The compiler provides __SIZE_TYPE__ as the type that corresponds to size_t
(typically "long unsigned int" or "unsigned int"). It was verified to be
available at least since gcc-3.4 and clang-3.8, so from now on we'll use
this definition for size_t.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/lkml/20230805161929.GA15284@1wt.eu/


Signed-off-by: Willy Tarreau <w@1wt.eu>