[ELF] - Implement the TLS relocation optimization for 32-bit x86.

Implement the TLS relocation optimization for 32-bit x86 that is described in
"ELF Handling For Thread-Local Storage" by Ulrich Drepper, chapter 5,
"IA-32 Linker Optimizations". Specifically, this patch implements these
optimizations: LD->LE, GD->IE, GD->LD, and IE->LE.

Differential revision: http://reviews.llvm.org/D15292

llvm-svn: 255103