[ELF] Add support for reading dynamic libraries.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@174916 91177308-0d34-0410-b5e6-96231b3b80d8
[Core,Driver,ELF] Differentiate static and dynamic executables.

This also adds a simple relocation change for dynamic executables to
x86-64 ELF.
Kinda sorta write out dynamic stuff.