[llvm-objcopy][docs] Add missing options to the help output and the command guide

This change is to keep the help text and command guide of objcopy in
tandem.

- In the help output the options --rename-section and
  --set-section-flags were missing the flag exclude, which is found in
  the command guide.
- In the command guide the alias -G for --keep-global-symbol was
    missing, which is found in the help output.

Differential Revision: https://reviews.llvm.org/D110340