kvm tools: Add initial SPAPR PPC64 architecture support

This patch adds a new arch directory, powerpc, basic file structure, register
setup and where necessary stubs out arch-specific functions (e.g. interrupts,
runloop exits) that later patches will provide.  The target is an
SPAPR-compliant PPC64 machine (i.e. pSeries); there is no support for PPC32 or
'bare metal' PPC64 guests as yet.  Subsequent patches implement the hcalls and
RTAS required to boot SPAPR pSeries kernels.

Memory is mapped from hugetlbfs (as that is currently required by upstream PPC64
HV-mode KVM).  The mapping of a VRMA region is yet to be implemented; this is
only necessary on processors that don't support VRMA, e.g. <= P6.  Work is
therefore needed to get this going on pre-P7 CPUs.

Processor state is set up as a guest kernel would expect (both primary and
secondaries), and SMP is fully supported.

Finally, support is added for simply loading flat binary kernels (plus initrd).
(bzImages are not used on PPC, and this series does not add zImage support or an
ELF loader.)  The intention is to later support loading firmware such as SLOF.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Pekka Enberg <penberg@kernel.org>