Support IBM System z s390 cpu

In IBM System z S390 archtecture,  the /proc/cpuinfo has totally different format with the x86 format. 
It looks like this:

```
# cat /proc/cpuinfo
vendor_id       : IBM/S390
<# processors    : 2
bogomips per cpu: 9328.00
features	: esan3 zarch stfle msa ldisp eimm dfp etf3eh highgprs 
processor 0: version = FF,  identification = 06E276,  machine = 2818
processor 1: version = FF,  identification = 06E276,  machine = 2818
```

So, we need to add additional code to make this s390 cpuinfo be resolved.
I think we treat the node attributes as a stable API, so putting the vendor_id somewhere else than the existing location seems likely an unnecessary change to that API, which feels worse than duplicating that data across multiple CPUs.

It's really unlikely this is going to be a problem because I don't know why anyone would care what's in that attribute, but people come up with lots of reasons to care about the value of attributes. 

That said, it's apparently already a mess because on OS X I get node[:cpuinfo][:vendor_id] = "GenuineIntel\n" and on Ubuntu I get node[:cpuinfo][:vendor_id][:1] = "GenuineIntel".

You probably can't have a motherboard with multiple processors from multiple vendors anyway, so I'm convinced I'm wasting our time talking about it now.