[LPM] Remove a PPC64 hack to try to work around a bad interaction

[LPM] Remove a PPC64 hack to try to work around a bad interaction
between the linker's TLS optimizations and Clang's TLS code generation.

For now, Clang has been changed to disable linker TLS optimizations
until it (and LLVM more generally) are emitting TLS code sequences
compatible with the old bugs found in the linkers. That's a better fix
to handle bootstrapping on that platform.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227511 91177308-0d34-0410-b5e6-96231b3b80d8