[foreign-abi] RFR 8235258: Add support for empty segments and rebased addresses

[Maurizio Cimadamore]

This patch implements the approach discussed in [1, 2] - which aims at 
increasing the range of 'safe' operations provided by the memory access API.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8235258_v2/

Changes compared to the patch in [2]:

* added support for all ABIs (please double check :-))
* refactored some instance methods in MemoryAddressImpl as static 
methods in Utils (as per Jorn suggestion)
* removed bound check from MemoryAddress::rebase (as per Jorn suggestion)
* added a combinatorial test which checks several cases of rebase(), 
more specifically:
   - for overlapping segments, it tests that accessing elements in the 
smaller segment works ok, regardless of whether the address used is 
rebased or not
   - for non-overlapping segments, if the segments have different kinds 
(e.g. heap vs. off-heap) check that an exception is thrown when rebase 
is called
   - for non-overlapping segments of the same kind check that an 
exception is thrown when the segment is accessed using the rebased 
address (as that should be OOB)

Maurizio


[1] - 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-November/006746.html 

[2] - 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-December/006780.html