[ELF] - Use error() instead of fatal() during relaxation of R_X86_64_GOTTPOFF

This is last known noticable fatal() in target.cpp.
We also have other ones for unknown relocations or
creating unknown targets, but that one can be just error I think.

Used yaml2obj to generate test.

Differential revision: https://reviews.llvm.org/D28049

llvm-svn: 290335