RFR(XS): 8053995: Add method to WhiteBox to get vm pagesize.

[Yumin Qi]

Please review:

http://cr.openjdk.java.net/~minqi/8053995/webrev00/

Summary: Currently there is no java API to get underlying OS native VM 
page size unless using Unsafe which is not recommended.  The new added 
method to WhiteBox can read this property and used in test.


Tests: JPRT and  jtreg.

Thanks
Yumin