[PowerPC] Work around TLS linker bug

Work around a bug in GNU ld (and gold) linker versions up to 2.25
that may mis-optimize code generated by this version of clang/LLVM
to access general-dynamic or local-dynamic TLS variables.

Bug is fixed here:
https://sourceware.org/ml/binutils/2015-01/msg00318.html

llvm-svn: 227480