Fix size_t for certain versions of GCC

When compiling coreboot with the latest ChromeOS toolchain, GCC
complains that some printk calls use %zu in connection with size_t
types since it resolves the typedefs to long unsigned int.

The problem is solved by using the GCC built-in __SIZE_TYPE__ if it
exists and define __SIZE_TYPE__ to long unsigned int otherwise.

Change-Id: I449c3d385b5633a05e57204704e981de6e017b86
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/1040
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

src/arch/x86/include/stddef.h

@@ -2,7 +2,10 @@
 #define I386_STDDEF_H
 
 typedef long ptrdiff_t;
-typedef unsigned long size_t;
+#ifndef __SIZE_TYPE__
+#define __SIZE_TYPE__ unsigned long
+#endif
+typedef __SIZE_TYPE__ size_t;
 typedef long ssize_t;
 
 typedef int wchar_t;