Fix PPC970 boot after r348783

r348783 changed the behavior of the kernel mappings and broke booting on G5.

• Split the kernel mapping logic out so that the case where we are

running from the wrong memory space is handled using identity
mappings, and the case where we are not using a DMAP is handled by
forcibly mapping the kernel into the dmap range as intended by
r348783.

Reported by: Mikael Urankar
Reviewed by: luporl
Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D20608