[foreign-abi] RFR: 8248499: Add methods to allocate off heap arrays from Java arrays [v2]

[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`).

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Add extra method to NativeScope to allocate (empty) arrays with arbitrary element layouts

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Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/222/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/222/files/ac6e37dc..38c81345

Webrevs:
 - full: https://webrevs.openjdk.java.net/panama-foreign/222/webrev.01
 - incr: https://webrevs.openjdk.java.net/panama-foreign/222/webrev.00-01

  Stats: 21 lines in 1 file changed: 19 ins; 2 del; 0 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