Please review fix for 6716785: implicit null checks not triggering with CompressedOops

[Coleen Phillimore - Sun Microsystems]

6716785: implicit null checks not triggering with CompressedOops
Summary: allocate alignment-sized page(s) below java heap so that memory 
accesses at heap_base+1page give signal and cause an implicit null check
Reviewed-by:

Webrev:
http://webrev.invokedynamic.info/coleenp/6716785/

Note in this webrev, the change in arguments.cpp to enable 
UseCompressedOops by default will be reverted for checking into HS13 
(it's still off by default) for 6u6p.  Should I enable it in our HS14 
source base?

Thanks!
Coleen