That should fix the Bug 26995 - [regression] unrecognized reloc 42 (https://llvm.org/bugs/show_bug.cgi?id=26995).
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.
Patch implementing relaxations was posted before and on review now (http://reviews.llvm.org/D15779).
There is no testcases provided as I think there is no way to generate such relocations using llvm-mc atm.