[yaml2obj/obj2yaml] - Do not trigger llvm_unreachable when dumping/parsing...

[yaml2obj/obj2yaml] - Do not trigger llvm_unreachable when dumping/parsing relocations and e_machine is unsupported.

Currently when e_machine is set to something that is not supported by YAML lib,
then tools fail with llvm_unreachable.

In this patch I allow them to handle relocations in this case.
It can be used to dump and create objects for broken or unsupported targets.

Differential revision: https://reviews.llvm.org/D67657

llvm-svn: 372377