KVM: PPC: Book3S HV P9: Implement PMU save/restore in C

Implement the P9 path PMU save/restore code in C, and remove the
POWER9/10 code from the P7/8 path assembly.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211123095231.1036501-14-npiggin@gmail.com