Make portage.util.compression_probe work when ctypes is unavailable

This is useful for bootstrapping purposes, as _ctypes requires a dynamic
linker.

Closes: #1363
Signed-off-by: Mike Gilbert <floppym@gentoo.org>

lib/portage/util/compression_probe.py

@@ -1,13 +1,13 @@
 # Copyright 2015-2020 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
-import ctypes
 import errno
 import re
 
 
 from portage import _encodings, _unicode_encode
 from portage.exception import FileNotFound, PermissionDenied
+from portage.util._ctypes import ctypes
 
 _compressors = {
     "bzip2": {
@@ -49,7 +49,7 @@
         # if the current architecture can support it, which is true when
         # sizeof(long) is at least 8 bytes.
         "decompress": "zstd -d"
-        + (" --long=31" if ctypes.sizeof(ctypes.c_long) >= 8 else ""),
+        + (" --long=31" if ctypes and ctypes.sizeof(ctypes.c_long) >= 8 else ""),
         "package": "app-arch/zstd",
     },
 }