[PowerPC] Add a FIXME.

Documenting a design choice to generate only medium model sequences for TLS
addresses at this time.  Small and large code models could be supported if
necessary.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190883 91177308-0d34-0410-b5e6-96231b3b80d8