[ELF][Hexagon] add initial changes to add GOT/PLT

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@176415 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF] rename Writer to ExecutableWriter

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@176416 91177308-0d34-0410-b5e6-96231b3b80d8