RFR(XS): Align up matrix size to page boundary

[Zhengyu Gu]

Run fastdebug: java -XX:+UseShenandoahGC 
-XX:ShenandoahGCHeuristics=partial -version

failed with:

#  Internal Error 
(/home/zgu/workspace/shenandoah-head/jdk9/hotspot/src/share/vm/memory/virtualspace.cpp:102), 
pid=25744, tid=25745
#  assert((size & (granularity - 1)) == 0) failed: size not aligned to 
os::vm_allocation_granularity()
#


Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/matrix_size/webrev.00/

Test:
   fastdebug and release: hotspot_gc_shenandoah


Thanks,

-Zhengyu