[llvm] [llvm-objcopy] Add llvm-objcopy option --set-symbol-visibility (PR #80872)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Add an option to manually set the visibility of symbols.
>
> There is already an option to set the visibility of newly added symbols via --add-symbol and --new-symbol-visibility. This option will allow to change the visibility of already existing symbols.

The first comment will be used as the default commit message. It needs to mention `--set-symbols-visibility`, so that people just searching for the option name can find this commit.

https://github.com/llvm/llvm-project/pull/80872