powerpc/64: move account_stolen_time into its own function

This will be used by interrupt entry as well.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210130130852.2952424-38-npiggin@gmail.com