[foreign-abi] RFR 8237762: Add platform dependent MemoryLayout constants

[Jorn Vernee]

Hi,

Please review the following patch that adds platform/ABI dependent 
MemoryLayout constants to MemoryLayouts.

Bug: https://bugs.openjdk.java.net/browse/JDK-8237762
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8237762/webrev.00/

Thanks,
Jorn