[llvm-objcopy] Add support for .dynamic, .dynsym, and .dynstr

This change adds support for sections involved in dynamic loading such
as SHT_DYNAMIC, SHT_DYNSYM, and allocated string tables.

The two added binaries used for tests can be downloaded [[
https://drive.google.com/file/d/0B3gtIAmiMwZXOXE3T0RobFg4ZTg/view?usp=sharing
| here ]] and [[
https://drive.google.com/file/d/0B3gtIAmiMwZXTFJSQUJZMGxNSXc/view?usp=sharing
| here ]]

Differential Revision: https://reviews.llvm.org/D36560

llvm-svn: 313663