mksysinfo: use rlimit64 if available when we use getrlimit64

mksysinfo: use rlimit64 if available when we use getrlimit64

    This makes no difference on most systems, because <sys/resource.h>
    renames the type appropriately anyhow, but apparently it makes a
    difference on AIX.

    Reviewed-on: https://go-review.googlesource.com/88076

From-SVN: r256877