[foreign-abi] RFR 8237761: Add way to determine the current ABI

[Jorn Vernee]

Hi,

Please review this small patch that adds a SystemABI::name() method that 
can be used to identify a SystemABI instance.

This is for instance useful when needing to select a MemoryLayout based 
on the currently used ABI, but doesn't require checking the type of the 
ABI against an internal type.

Bug: https://bugs.openjdk.java.net/browse/JDK-8237761
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8237761/webrev.00/

Thanks,
Jorn