Turn on UseNUMA by default when prudent

[Eric Caspole]

Hi Hotspot team,

We sometimes notice on multi-socket systems large run to run  
variation in some benchmarks which seems to be attributed to bad/ 
unlucky numa behavior. So we would like to add simple checks to turn  
on +UseNUMA for AMD platforms in the right situations; for example,  
max heap >= 4GB and we detect a multi-socket system. This should be a  
useful out-of-box improvement for most customers since UseParallelGC  
is the default.

For benchmarks with a large heap, it helps give better out-of-box  
performance in many cases.

Here is my first webrev to turn on NUMA for AMD systems when it is a  
numa system and the heap is set >= 4GB.

  http://cr.openjdk.java.net/~ecaspole/numa_default/

Please let me know your thoughts on the merit of doing this and if it  
is worthwhile what should be the structure of the change in the  
shared/os/cpu specific code.

Thanks,
Eric