OAv2 merge: merge subsections and tags at file scope

OAv2 merge: merge subsections and tags at file scope

Before merging input object files together, each input's object
attributes must be sanitized to ensure that duplicate subsections and
attributes are merged and properly sorted.  Indeed, uniqueness and
ordering of subsections and attributes are required prerequisites for
the merge process.
This operation also handles, on a best-effort basis, exotic objects
produced by non-GNU assemblers, where duplicate subsections may appear
across different sections, and similarly, duplicate attributes across
subsections.
It is importand to note that such cases are not covered by the Object
Attributes specification, and the resulting behavior is implementation
-defined.  Other linkers may handle these inputs differently.