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/stdio/vcscanf.c

@@ -44,8 +44,11 @@
  * @param va points to the variadic argument state
  * @see libc/fmt/pflink.h (dynamic memory is not a requirement)
  */
-int vcscanf(int callback(void *), int unget(int, void *), void *arg,
-            const char *fmt, va_list va) {
+int __vcscanf(int callback(void *),    //
+              int unget(int, void *),  //
+              void *arg,               //
+              const char *fmt,         //
+              va_list va) {
   struct FreeMe {
     struct FreeMe *next;
     void *ptr;