This adds --hide-symbol and --hide-symbols options to elfcopy, which give the given symbol(s) hidden visibility.
I used this during lld bringup, before it supported versioned symbols, to build a version of libc_pic.a with symbols hidden, so that they didn't leak into ld-elf.so. (Also see D5650 for another change related to disabling symbol versioning.)
Versioned symbols are now supported in lld and I no longer need this, but it seems like a feature that could be useful to have in elfcopy, so I'm posting for comments. Man page will be updated if there's interest in taking this.