follow dpkg and xz and use CRC64 for xz compression

dpkg switched from CRC32 to CRC64 in
777915108d9d36d022dc4fc4151a615fc95e5032 with the message:
| This is the default CRC used by the xz command-line tool, align with
| it and switch from CRC32 to CRC64. It should provide slightly better
| detection against damaged data, at a negligible speed difference.

apt-pkg/contrib/fileutl.cc

@@ -1278,7 +1278,7 @@ public:
       {
 	 if (compressor.Name == "xz")
 	 {
-	    if (lzma_easy_encoder(&lzma->stream, xzlevel, LZMA_CHECK_CRC32) != LZMA_OK)
+	    if (lzma_easy_encoder(&lzma->stream, xzlevel, LZMA_CHECK_CRC64) != LZMA_OK)
 	       return false;
 	 }
 	 else