take control over package versions for Prefix bootstrap

[boegel]

To take control over the exact version that is installed of important packages that are installed in the compat layer when performing the Prefix bootstrap, we can use a package.mask file; see https://wiki.gentoo.org/wiki/Handbook:AMD64/Full/Portage#Masked_packages.

This could be important for glibc for example, since using the bleeding-edge version of glibc could cause problems in the software layer, we should probably be a bit more conservative there.

There are known problems with glibc 2.34 (current latest) and the combination of CUDA and GCC 11.x, for example (@amadio: any references on that)?

[amadio]

https://forums.developer.nvidia.com/t/cuda-11-5-samples-throw-multiple-error-attribute-malloc-does-not-take-arguments/192750

[amadio]

The bad binutils version has been masked by Sam on prefix. As for glibc, I think that for you it's probably a good idea to mask 2.34 until a newer CUDA has a fix for the problem above. So I believe that with glibc 2.33 and the masked binutils, you should be able to bootstrap now.

[bedroge]

Some advice from @amadio on Slack:

• disable the sync that the bootstrap script does by removing it or by setting auto-sync = no for the gentoo overlay; the latter can be done before starting the bootstrap, but make sure that it doesn' get overwritten.
• make/use a custom snapshot
• keep in mind that the layers for the different architectures can still install different versions due to the keywording of packages!
• test versions using qlist -ICv