Make a change to the implementation of the direct map to improve performance

Make a change to the implementation of the direct map to improve performance
on processors that support 1 GB pages.  Specifically, if the end of physical
memory is not aligned to a 1 GB page boundary, then map the residual
physical memory with multiple 2 MB page mappings rather than a single 1 GB
page mapping.  When a 1 GB page mapping is used for this residual memory,
access to the memory is slower than when multiple 2 MB page mappings are
used.  (I suspect that the reason for this slowdown is that the TLB is
actually being loaded with 4 KB page mappings for the residual memory.)

X-MFC after:	r214425