Allocate vm_page_array and vm_page_buckets from the end of the biggest chunk of memory, rather than from the start.
Allocate vm_page_array and vm_page_buckets from the end of the biggest chunk
of memory, rather than from the start.

This fixes problems allocating bouncebuffers on alphas where there is only
1 chunk of memory (unlike PCs where there is generally at least one small
chunk and a large chunk).  Having 1 chunk had been fatal, because these
structures take over 13MB on a machine with 1GB of ram. This doesn't leave
much room for other structures and bounce buffers if they're at the front.

Reviewed by: dfr, anderson@cs.duke.edu, silence on -arch
Tested by: Yoriaki FUJIMORI <fujimori@grafin.fujimori.cache.waseda.ac.jp>

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase