[llvm-objcopy] Add support for --rename-section flags from gnu objcopy

Summary:
Add support for --rename-section flags from gnu objcopy.

Not all flags appear to have an effect for ELF objects, but allowing them would allow easier drop-in replacement. Other unrecognized flags are rejected.

This was only tested by comparing flags printed by "readelf -e <.o>" against the output of gnu vs llvm objcopy, it hasn't been tested to be valid beyond that.

Reviewers: jakehehrlich, alexshap

Subscribers: llvm-commits, paulsemel, alexshap

Differential Revision: https://reviews.llvm.org/D49870

llvm-svn: 338582