stddef.h: Introduce nullptr constant

GCC-13 introduced the nullptr constant. Use it when compiling with the
C23 standard.

Change-Id: I07db866bebfd25f1a60d18a3228ada2957500234
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83459
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/stddef.h

@@ -19,7 +19,12 @@ typedef __SIZE_TYPE__ ssize_t;
 typedef __WCHAR_TYPE__ wchar_t;
 typedef __WINT_TYPE__ wint_t;
 
+#if __STDC_VERSION__ >= 202300L
+#define NULL nullptr
+#else
+#define nullptr ((void *)0)
 #define NULL ((void *)0)
+#endif
 
 /* The devicetree data structures are only mutable in ramstage. All other
    stages have a constant devicetree. */