[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
Make lld-link symlink relative.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220130 91177308-0d34-0410-b5e6-96231b3b80d8
Sort include files according to convention.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220131 91177308-0d34-0410-b5e6-96231b3b80d8