mark our ebuilds as stable

[boegel]

While testing the installation of the 2021.12 compat layer on x86_64, I ran into this problem:

All ebuilds that could satisfy "=sys-apps/archspec-0.1.3::eessi" have been masked.
!!! One of the following masked packages is required to complete your request:
- sys-apps/archspec-0.1.3::eessi (masked by: ~amd64 keyword)

This is caused by the new "stable" mode of the Prefix bootstrap (see also EESSI/compatibility-layer#131), in combination with not tagging our packages as stable (by using amd64 rather than ~amd64 for archspec, for example).