RFR(XS): 8226302: Test failures on IBM platforms (power and s/390) after JDK-8223837

[Doerr, Martin]

Hi,

I'd like to fix the heap size related issues described in
https://bugs.openjdk.java.net/browse/JDK-8226302
by adjusting the sizes. I prefer this simple approach for jdk13.

We could change the tests such that they determine appropriate sizes depending on the HeapAlignment (retrieved by whitebox api) for jdk14.

Please review:
http://cr.openjdk.java.net/~mdoerr/8226302_TestHeapSize/webrev.00/

Best regards,
Martin