[llvm-objcopy] Add basic support for --rename-section

Summary:
Add basic support for --rename-section=old=new to llvm-objcopy.

A full replacement for GNU objcopy requires also modifying flags (i.e. --rename-section=old=new,flag1,flag2); I'd like to keep that in a separate change to keep this simple.

Reviewers: jakehehrlich, alexshap

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49576

llvm-svn: 337604