[foreign-abi] RFR: 8248499: Add methods to allocate off heap arrays from Java arrays

[Maurizio Cimadamore]

This patch adds methods to NativeScope which allow clients to allocate off-heap arrays from Java on-heap arrays.

While in most cases the code for copying is straightforward, we have to take into account endianness mismatches (in
case the user wants to store the array elements off-heap in an endianness that doesn't match that of the platform).

To allow for that, I've added a new implementation method in AbstractMemorySegmentImpl, which is the same as
`copyFrom`, but adds the byte swap (a wrapper around `Unsafe::copySwapMemory`).

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Commit messages:
 - Remove whitespaces
 - Add test

Changes: https://git.openjdk.java.net/panama-foreign/pull/222/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/222/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248499
  Stats: 406 lines in 3 files changed: 349 ins; 9 del; 48 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/222.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/222/head:pull/222

PR: https://git.openjdk.java.net/panama-foreign/pull/222