KVM: PPC: Add helpers to call FPU instructions

To emulate paired single instructions, we need to be able to call FPU
operations from within the kernel. Since we don't want gcc to spill
arbitrary FPU code everywhere, we tell it to use a soft fpu.

Since we know we can really call the FPU in safe areas, let's also add
some calls that we can later use to actually execute real world FPU
operations on the host's FPU.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>