add defaults for compat layer variant symlinks

[bedroge]

Accompanying PR for EESSI/compatibility-layer#235, this creates the defaults symlinks for compat layer variant symlinks.

As an example, we get this structure:

/cvmfs/software.eessi.io/versions/2026.06/compat/linux/x86_64/etc/hosts
-> 
'$(EESSI_202606_COMPAT_ETC_HOSTS):-/cvmfs/software.eessi.io/defaults/compat/etc/hosts'
->
'$(EESSI_COMPAT_ETC_HOSTS):-/etc/hosts'

I.e., by default every symlink will point to the same file on the host, but you can override the default (i.e. for all EESSI versions) or per version.

[bedroge]

Should these compat variables also have _DEFAULT, i.e. EESSI_COMPAT_ETC_ETHERS_DEFAULT? I figured the name would be clear enough without it, it's already in a defaults directory anyway, but for consistency we could add it.