boot: add Provides:systemd-boot(isa)
    
    As requested in https://github.com/rhinstaller/anaconda/pull/4368#discussion_r1043839809,
    so that it's easier to depend on the appropriate package. Once we have the
    signed version built, this provides might be dropped. But let's add it at least
    for now so that there's a stable name to depend on.
    
    While at it, let's drop ? from %{_isa}. Systemd is always archful.