[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:
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