Use Endian.h to simplify this code a bit.

While at it, factor some logic into FragmentWriter. This will allow more code
to be factored out of the fairly large ELFObjectWriter.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204765 91177308-0d34-0410-b5e6-96231b3b80d8