KVM: PPC: Book3S PR: Add C function wrapper for _kvmppc_save/restore_tm()

Currently __kvmppc_save/restore_tm() APIs can only be invoked from
assembly function. This patch adds C function wrappers for them so
that they can be safely called from C function.

Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>