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/vprintf.c

@@ -22,6 +22,6 @@
  * Formats and writes text to stdout.
  * @see printf() for further documentation
  */
-int(vprintf)(const char* fmt, va_list va) {
-  return (vfprintf)(stdout, fmt, va);
+int vprintf(const char* fmt, va_list va) {
+  return vfprintf(stdout, fmt, va);
 }