powerpc/booke: Add pte_find_next() to find the next in-use PTE

[jhibbits]

Summary

Iterating over VM_MIN_ADDRESS->VM_MAXUSER_ADDRESS can take a very long
time iterating one page at a time (2**(log_2(SIZE)-12) operations),
yielding possibly several days or even weeks on 64-bit Book-E, even for
a largely empty, which can happen when swapping out a process by
vmdaemon. Speed this up by instead finding the next PTE at or equal to
the given VA.

[jhibbits]

Fix the build

[jhibbits]

Fix 32-bit boot. Tested in qemu.

[bdragon]

Works on X5000 and RB800.