[lib/ObjectYAML] - Make `ELFYAML::Relocation::Offset` optional.

Currently `yaml2obj` require `Offset` field in a relocation description.
There are many cases when `Offset` is insignificant in a context of a test case.

Making `Offset` optional allows to simplify our test cases.
This is what this patch does.

Also, with this patch `obj2yaml` does not dump a zero offset of a relocation.

Differential revision: https://reviews.llvm.org/D75608