[llvm-objcopy] Don't remove .gnu_debuglink section when using --strip-all (#78919)

This fixes the issue mentioned here:
https://github.com/llvm/llvm-project/issues/57407
It prevents `llvm-objcopy` from removing the `.gnu _debuglink` section
when used with the `--strip-all` flag. Since `--strip-all` is the
default of `llvm-strip` the patch also prevents `llvm-strip` from
removing the `.gnu_debuglink` section.