[llvm-objcopy] - Strip undefined symbols if they are no longer referenced following --only-section

This is https://bugs.llvm.org/show_bug.cgi?id=40004.

In this patch I teach llvm-objcopy to remove undefined symbols if
them are not used anymore after applying -j/--only-section option.

Differential revision: https://reviews.llvm.org/D62317

llvm-svn: 361642