initial implementation of klibc

fix linker error about ctx_switch

klibc/include/stddef.h

@@ -0,0 +1,10 @@
+#ifndef CLASSICOS_KLIBC_STDDEF_H
+#define CLASSICOS_KLIBC_STDDEF_H
+
+typedef __SIZE_TYPE__ size_t;
+typedef __PTRDIFF_TYPE__ ptrdiff_t;
+
+#undef NULL
+#define NULL ((void*)0)
+
+#endif  // CLASSICOS_KLIBC_STDDEF_H