[ELF] - Process R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX relocations.

R_X86_64_GOTPCRELX and R_X86_64_REX_GOTPCRELX relocations were added in latest ABI:
https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-r249.pdf

They should be generated instead of R_X86_64_GOTPCREL for cases 
when relaxation is possible. Currently this patch just process them in the 
same way like R_X86_64_GOTPCREL. That should work for now
and we can implement relaxations later.

There is no testcases provided as I think there is no way to generate
such relocations using llvm-mc atm.

Differential revision: http://reviews.llvm.org/D18301

llvm-svn: 264043