Use status to implement file_size.

The status function is already using a syscall that returns the file size.
Remember it and implement file_size as a simple wrapper.

No functionally change, but clients that already use status now can avoid
calling file_size.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186016 91177308-0d34-0410-b5e6-96231b3b80d8