kernel: add ubsan bindings, fix some UB reported by it

kernel-src/include/string.h

@@ -7,7 +7,7 @@ size_t strlen(const char *str);
 char *strcpy(char *dest, const char *src);
 char *strcat(char *dest, const char *str);
 void *memcpy(void *dest, const void *src, size_t size);
-void *memset(void *dest, unsigned long what, size_t size);
+void *memset(void *dest, int what, size_t size);
 int strcmp(const char *a, const char *b);
 int strncmp(const char *a, const char *b, size_t c);
 int memcmp(const void *a, const void *b, size_t s);