[ELF] - Implemented @tlsgd optimization (GD->IE case, x64).

"Ulrich Drepper, ELF Handling For Thread-Local Storage" (5.5 x86-x64 linker optimizations, http://www.akkadia.org/drepper/tls.pdf) shows how GD can be optimized to IE.
This patch implements the optimization.

Differential revision: http://reviews.llvm.org/D15000

llvm-svn: 254713