[lld][ELF] Rename typeTLV content type for ELF

typeTLV content type is used by Darwin to represent thread local
storage. A new contentType has to be made to represent ELF
thread local storage data. These have been set to

- typeThreadZeroFill (represents TBSS storage)
- typeThreadData     (represents TDATA storage)

llvm-svn: 189137