[ELF] - Fixed handling R_X86_64_DTPOFF64 relocation relaxation

R_X86_64_DTPOFF64 was not handled properly.
Next sample app was impossible to link before this patch:

~/pg/release/bin/clang -target x86_64-pc-linux testthread.cpp -c -g
~/pg/d+a/bin/ld.lld testthread.o 
"Unknown TLS optimization" (value was 17)

__thread int x = 0;
void _start() {
}

It works fine now.

Differential revision: http://reviews.llvm.org/D18039

llvm-svn: 263150