[ELF] Add Readers for all the ELF subtargets.

This would permit the ELF reader to check the architecture that is being
selected by the linking process.

This patch also sorts the include files according to LLVM conventions.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220129 91177308-0d34-0410-b5e6-96231b3b80d8