Add ELF dynamic symbol support to yaml2obj/obj2yaml

Summary:
This change introduces a `DynamicSymbols` field to the ELF specific YAML
supported by `yaml2obj` and `obj2yaml`. This grouping of symbols provides a way
to represent ELF dynamic symbols. The `DynamicSymbols` structure is identical to
the existing `Symbols`.

Reviewers: compnerd, jakehehrlich, silvas

Reviewed By: silvas

Subscribers: silvas, jakehehrlich, llvm-commits

Differential Revision: https://reviews.llvm.org/D39582

llvm-svn: 318433