PR16493: DebugInfo with TLS on PPC crashing due to invalid relocation

Restrict the current TLS support to X86 ELF for now. Test that we don't
produce it on PPC & we can flesh that test case out with the right thing
once someone implements it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185389 91177308-0d34-0410-b5e6-96231b3b80d8