perf/powerpc: Use perf_arch_instruction_pointer()

Make sure PowerPC uses the arch-specific function now that those have
been reorganized.

Signed-off-by: Colton Lewis <coltonlewis@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Acked-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://lore.kernel.org/r/20241113190156.2145593-4-coltonlewis@google.com