[NTOS]: Define the POOL_HEADER for x64.

[NTOS]: Define the POOL_HEADER for x64.
[NTOS]: Define POOL_BLOCK_SIZE definition to set the minimum pool block size. In NT, this is equal to a LIST_ENTRY structure, because the Pool Allocator must be able to store a LIST_ENTRY into a freed pool block. This also determines the alignment of pool allocations. So 8 on x86, 16 on x64.
[NTOS]: Don't depend on LIST_ENTRY, but use POOL_BLOCK_SIZE instead (on IA64, if we ever want to support this, the pool block size is different from a LIST_ENTRY/POOL_HEADER).
[NTOS]: The following ASSERTs must hold: the POOL_HEADER must be as big as the the smallest pool block (POOL_BLOCK_SIZE), which must be at least as big as a LIST_ENTRY structure. 8 == 8 == 8 on x86, 16 == 16 == 16 on x64.

svn path=/trunk/; revision=47592