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/dprintf.c

@@ -21,11 +21,11 @@
 /**
  * Formats string directly to file descriptor.
  */
-int(dprintf)(int fd, const char *fmt, ...) {
+int dprintf(int fd, const char *fmt, ...) {
   int rc;
   va_list va;
   va_start(va, fmt);
-  rc = (vdprintf)(fd, fmt, va);
+  rc = vdprintf(fd, fmt, va);
   va_end(va);
   return rc;
 }