[PowerPC] Fix kernel boot on powerpc

Recent changes have caused the vmspace objects to start coming from KVA
instead of direct-mapped memory on powerpc. As far as I can tell, this is
not actually a problem, so we should stop arbitrarily asserting that it is.

I do not know why this was not being triggered before.

Approved by: jhibbits
Sponsored by: Tag1 Consulting, Inc.