Fix brp-strip-static-archive parallelism
    
    The change made in fc2c986 can break for large values of %_smp_build_ncpus as
    this many processes are able to overflow the following pipe.
    
    Thanks to Denys Vlasenko for testing this.
    
    This change solves this problem by running a whole processing pileline for each
    parallel (file) process. This has also the benefit of running at least some
    stip commands in parallel.
    
    The -n param fro xargs was increased to 32 to further reduce the over head of
    spawing the helpers as they are now needed for each run of the file command.