[ELF2] R_X86_64_64 plt relocation implemented.

This little patch is a final point in fix for https://llvm.org/bugs/show_bug.cgi?id=25175.

It`s make sample from issue description to work with -O0 optimization flag (with -O0 it generates R_X86_64_64 plt relocation).

Differential revision: http://reviews.llvm.org/D14153

llvm-svn: 251536