libgo: separate mksysinfo inputs into separate Makefile targets

    
    This is a step toward a version of mksysinfo that generates information
    for the runtime package.  This will be used to generate the
    runtime_sysinfo.go file, which is currently directly generated by a
    Makefile target.
    
    Reviewed-on: https://go-review.googlesource.com/29683

From-SVN: r240560