[ELF] Dont include all targets

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200165 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF] Rename File.h to ELFFile.h to avoid confusion.

No change in functionality.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200166 91177308-0d34-0410-b5e6-96231b3b80d8