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/vfprintf_unlocked.c

@@ -51,7 +51,7 @@ static int vfprintfputchar(const char *s, struct state *t, size_t n) {
  * Formats and writes text to stream.
  * @see printf() for further documentation
  */
-int(vfprintf_unlocked)(FILE *f, const char *fmt, va_list va) {
+int vfprintf_unlocked(FILE *f, const char *fmt, va_list va) {
   int rc;
   struct state st[1] = {{f, 0}};
   if ((rc = __fmt(vfprintfputchar, st, fmt, va)) != -1) {