gitlab-ci: replace '$os' tags with '$os-x86_64'
The latter tags are now available on our CI runners to more explicitly distinguish {linux,macos,windows}-x86_64
from linux-aarch64
and {macos,windows}-arm64
.
Also rename tag configuration entries to start in $os_$arch_tags
. This groups their definitions nicely, and makes their purpose more clear from reference sites.