Probe and report page sizes supported by MIPS hardware.

This change probes the hardware to see what page sizes are supported
by writing all ones to the PageMask register and reading it back as
suggested by "MIPS Architecture For Programmers Volume III".  The
page sizes the hardware supports is reported the same time the
information about the MMU/TLB is printed.

Sponsored by:       DARPA, AFRL
Move the machine dependent code parts out of vm_thread_new().

This change moves the machine dependent code for allocating memory,
for both virtual and physical, out of vm_thread_new().  The reason
this code is machine dependent is the memory has to be contiguous
and properly aligned for MIPS.

Sponsored by:       DARPA, AFRL
Add option to double the kernel thread stack by using larger page size.

This code change adds the kernel option KSTACK_LARGE_PAGE that uses one
16K page for the kernel stack instead of only 4K page sizes. This doubles
the kernel thread stack size without using additional wired TLB entries.
To use this "option KSTACK_LARGE_PAGE" must be added to the kernel conf
file.

Sponsored by:       DARPA, AFRL