bug#55806: gnu/system/install.scm includes (service uvesafb-service-type) in %installation-services on all architectures

[Pavel Shlyak]

Reproducer:
guix system image -t iso9660 gnu/system/install.scm
This command fails on aarch64 host. It doesn’t if I manually remove 
uvesafb-service-type from the list. It is impossible(and not needed) to run 
uvesafb on non-x86 machines at the moment, so I think it should be included 
only in x86_64 (and probably 32bit x86 as well) builds.