gpu/cpu mutex naming

cc @conda-forge/core

As we are adding more packages, we have to decide on the naming.

Currently most packages use `-proc=*=cpu` and `-proc=*=gpu`. While it is great to have a uniform naming scheme, I don't particularly like the `-proc` name since the name doesn't mean anything. We should decide on the naming soon.

We can fix already existing packages to work with the new name and the old name if we decide on a new name.