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

I overzealously landed this before I was sure that another change
wouldn't break the build that this change depends on.

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 here and here

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

llvm-svn: 313767