[ELF] Give a slightly better message for unknown relocation types.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@171692 91177308-0d34-0410-b5e6-96231b3b80d8
[Driver] x86-64 should use ELF 64.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@171693 91177308-0d34-0410-b5e6-96231b3b80d8