[lld] Move AtomLayout from ELF to ReaderWriter so that it can be used by non-ELF writers.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D977

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@184061 91177308-0d34-0410-b5e6-96231b3b80d8