powerpc64: Split out DMAP and non-DMAP implementations of some methods

[jhibbits]

Summary

Some methods are split between DMAP and non-DMAP, conditional on
hw_direct_map variable. Rather than checking this variable every time,
use it to install different functions via IFUNCs.

[luporl]

LGTM. The code is also easier to read now, without those hw_direct_map conditionals.