[yaml2obj] - Introduce the "Offset" property for sections.

Currently there is no good way to set a physical offset for a section:

* We have the `ShOffset` that allows to override the `sh_offset`, but
  it does not affect the real data written.
* We can use a `Filler` to create an artificial gap, but it is more like a hack
  rather than a proper solution for this problem.

This patch adds the `Offset` property which allows setting physical
offsets for sections.

It also generalizes the code, so that we set sh_offset field in one place

Differential revision: https://reviews.llvm.org/D78927