[MINOR] Enhance SizeEstimator to detect IBM compressed refs and s390 …

…arch.

 - zSeries 64-bit Java reports its architecture as s390x, so enhance the 64-bit check to accommodate that value.

 - SizeEstimator can detect whether IBM Java is using compressed object pointers using info in the "java.vm.info" property, so will do a better job than failing on the HotSpot MBean and guessing.

Author: Tim Ellison <t.p.ellison@gmail.com>

Closes #6085 from tellison/SizeEstimator and squashes the following commits:

1b6ff6a [Tim Ellison] Merge branch 'master' of https://github.com/apache/spark into SizeEstimator
0968989 [Tim Ellison] [MINOR] Enhance SizeEstimator to detect IBM compressed refs and s390 arch.