[yaml2obj] - Support the "Offset" key for the .dynsym section.

Our "implicit" sections are handled separately from regular ones.
It turns out that the "Offset" key is not handled properly for them.

Perhaps we can generalize handling in one place, but before doing that I'd like
to add support and test cases for each implicit section.
(I need this particular single change to unblock another patch that is already on review,
and I guess doing it independently for each section will be cleaner, see below).

In this patch I've removed `explicit-dynsym-no-dynstr.yaml` to `dynsym-section.yaml`
and added the new test into. In a follow-up we probably might want
to merge 2 another existent `dynsymtab-*.yaml` tests into it too.

Differential revision: https://reviews.llvm.org/D90224