RFR: 8151534: Refactor ArrayAllocator for easier reuse

[Stefan Karlsson]

Hi all,

Please review this patch to refactor the ArrayAllocator code.

http://cr.openjdk.java.net/~stefank/8151534/webrev.00
https://bugs.openjdk.java.net/browse/JDK-8151534

ArrayAllocator is used to either allocate an array with malloc or mmap. 
This patch extracts the allocation code into two separate classes: 
MmapArrayAllocator and MallocArrayAllocator. ArrayAllocator will 
dispatch to one of these classes depending on the size of the 
allocation. The main motivation for this patch is to allow the fix for 
JDK-8077144 to reuse the code in MmapArrayAllocator.

Thanks,
StefanK