Clean up some code

- Use good ELF technique in cosmo_dlopen()
- Make strerror() conform more to other libc impls
- Introduce __clear_cache() and use it in cosmo_dlopen()
- Remove libc/fmt/fmt.h header (trying to kill off LIBC_FMT)

third_party/python/Lib/test/test_doctest.py

@@ -2921,7 +2921,7 @@ Invalid file name:
     >>> print(normalize(err))                    # doctest: +ELLIPSIS
     Traceback (most recent call last):
       ...
-    FileNotFoundError: [Errno 2] ENOENT...
+    FileNotFoundError: [Errno 2] No such file or directory...
 
 Invalid doctest option: