[llvm-objcopy] Add support for dynamic relocations

This change adds support for dynamic relocations (allocated
SHT_REL/SHT_RELA sections with a dynamic symbol table as their link).

The binary I added for the test is here:
https://drive.google.com/file/d/0B3gtIAmiMwZXSjJUZE9pUjd4M0k/view?usp=sharing

Unless support for dynamic symbol tables in yaml2obj is added this is
needed.

Differential Revision: https://reviews.llvm.org/D37915

llvm-svn: 314227