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.
The new python script check_root uses data that portage already
maintains on what shared libraries packages need or provide instead of
re-scanning whatever ELF files that can be found. This is much more
comprehensive but there is a bit of a transition issue for folks with
long-lived SDKs: packages built with portage older than 2.2.18 do not
include this data. As such for now the check is non-fatal and provides a
command you can use to refresh locally installed packages.
The code checking for conflicts between top level directories and /usr
has also been rewritten. Both tests now are considerably faster.
