message: Only use sendfile implementation on linux.
  That is, use copy_file_linux.cc only if we are on linux. This fixes a bug where BSD sendfile is not the same as Linux sendfile so Mac builds fail.
diff: @@ -25,8 +25,8 @@ cc_library(
  "posix_filesystem_helper.h",
  "copy_file.h",
  ] + select({
  - "//tensorflow:android": ["copy_file_portable.cc"],
  - "//conditions:default": ["copy_file_linux.cc"],
  + "//tensorflow:linux_x86_64": ["copy_file_linux.cc"],
  + "//conditions:default": ["copy_file_portable.cc"],
  }),
  deps = [
  "//tensorflow/c:tf_status",
  