Clean up more code

- Found some bugs in LLVM compiler-rt library
- The useless LIBC_STUBS package is now deleted
- Improve the overflow checking story even further
- Get chibicc tests working in MODE=dbg mode again
- The libc/isystem/ headers now have correctly named guards

libc/runtime/utmp.c

@@ -39,6 +39,10 @@ void updwtmp(const char *x, const struct utmp *y) {
 void updwtmpx(const char *x, const struct utmpx *y) {
 }
 
+struct utmpx *pututxline(const struct utmpx *p) {
+  return 0;
+}
+
 void setutxent(void) {
 }