Address the issue mentioned in [1] regarding the core (xnheap)
allocator, using a variant of the McKusick scheme significantly
improving the performance figures.

As a by-product of this overhaul, the core allocator can now manage
heaps up to (4GB - PAGE_SIZE).

The performance report log obtained by testing on imx6qp
is as follows:

== memcheck started
     seq_heap_size=2048k
     random_alloc_rounds=1024
     pattern_heap_size=128k
     pattern_check_rounds=128

[SEQUENTIAL ALLOC->FREE, ^2 BLOCK SIZES] ON 'xnheap'

HEAPSZ	test heap size
BLOCKSZ	tested block size
NRBLKS	number of blocks allocatable in heap
AVG-A	average time to allocate block (us)
AVG-F	average time to free block (us)
MAX-A	max time to allocate block (us)
MAX-F	max time to free block (us)
FLAGS	+shuffle: randomized free
    	+realloc: measure after initial alloc/free pass (hot heap)

sorted by: max alloc time
  HEAPSZ  BLOCKSZ   NRBLKS  AVG-A  AVG-F  MAX-A  MAX-F   FLAGS
   1024k       32    32768      0      0      8      6
   1024k       32    32768      0      0      7      2   +shuffle +realloc
   1024k       16    65536      0      0      7      2   +realloc
   1024k       16    65536      0      0      6      7   +shuffle +realloc
  ... (364 results following) ...

sorted by: max free time
  HEAPSZ  BLOCKSZ   NRBLKS  AVG-A  AVG-F  MAX-A  MAX-F   FLAGS
   1024k      128     8192      0      1      2      8
   1024k       16    65536      0      0      6      7   +shuffle +realloc
   1024k       32    32768      0      0      8      6
    512k       32    16384      0      0      5      6   +realloc
  ... (364 results following) ...

overall:
  worst alloc time: 8 (us)
  worst free time: 8 (us)
  average of max. alloc times: 1 (us)
  average of max. free times: 2 (us)
  average alloc time: 1 (us)
  average free time: 1 (us)

[SEQUENTIAL ALLOC->FREE, RANDOM BLOCK SIZES] ON 'xnheap'

HEAPSZ	test heap size
BLOCKSZ	tested block size
NRBLKS	number of blocks allocatable in heap
AVG-A	average time to allocate block (us)
AVG-F	average time to free block (us)
MAX-A	max time to allocate block (us)
MAX-F	max time to free block (us)
FLAGS	+shuffle: randomized free
    	+realloc: measure after initial alloc/free pass (hot heap)

sorted by: max alloc time
  HEAPSZ  BLOCKSZ   NRBLKS  AVG-A  AVG-F  MAX-A  MAX-F   FLAGS
    512k       17k      28      1      1      8      2   +shuffle
    512k       45k      11      1      1      7      2
   1024k       24    32768      0      0      7      6   +shuffle
    128k      820      128      1      1      6      2   +shuffle
  ... (32764 results following) ...

sorted by: max free time
  HEAPSZ  BLOCKSZ   NRBLKS  AVG-A  AVG-F  MAX-A  MAX-F   FLAGS
   1024k        3k     292      1      1      1      8   +shuffle
    256k      174     1024      1      1      1      6   +shuffle
   1024k       24    32768      0      0      7      6   +shuffle
     32k       12k       2      2      3      1      5
  ... (32764 results following) ...

overall:
  worst alloc time: 8 (us)
  worst free time: 8 (us)
  average of max. alloc times: 1 (us)
  average of max. free times: 1 (us)
  average alloc time: 1 (us)
  average free time: 1 (us)

[1] http://www.xenomai.org/pipermail/xenomai/2018-April/038883.html