Kernel source tree started to have a broken link
`tools/testing/selftests/powerpc/copyloops/memcpy_mcsafe_64.S`.
Especially in case of Kernel 5.8.18, like:
```
broken link: /usr/src/linux-5.8.18-coreos/tools/testing/selftests/powerpc/copyloops/memcpy_mcsafe_64.S
ERROR build_packages: test_image_content: Failed symlink check
```
Ignore the symlink when checking broken symlinks.