[SDK] Minor improvements to CRT, RTL, and headers

Signed-off-by: Quinn Stephens <quinn@osmora.org>

SDK/INC/CRT/stddef.h

@@ -53,6 +53,8 @@ typedef long int ptrdiff_t;
 typedef unsigned short wchar_t;
 #endif
 
+#define offsetof(type, member) __builtin_offsetof(type, member)
+
 #ifdef __cplusplus
 }
 #endif