We use SIAR or regs->nip for the instruction pointer depending on
the PMU configuration, but we always use regs->nip in the callchain.

Use perf_instruction_pointer so the backtrace is consistent.
Signed-off-by: NAnton Blanchard <anton@samba.org>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>