[ELF] [X86_64] Update TODO.rst list wrt r218633

Updates the remaining tasks in the X86_64 ELF lld backend after the commit that
handles general dynamic TLS relocations.

Reviewer: shankarke

http://reviews.llvm.org/D5673


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@219350 91177308-0d34-0410-b5e6-96231b3b80d8