libbpf: Fix powerpc's stack register definition in bpf_tracing.h

libbpf: Fix powerpc's stack register definition in bpf_tracing.h

retsnoop's build on powerpc (ppc64le) architecture ([0]) failed due to
wrong definition of PT_REGS_SP() macro. Looking at powerpc's
implementation of stack unwinding in perf_callchain_user_64() clearly
shows that stack pointer register is gpr[1].

Fix libbpf's definition of __PT_SP_REG for powerpc to fix all this.

  [0] https://kojipkgs.fedoraproject.org/work/tasks/1544/137921544/build.log

Fixes: 138d6153a139 ("samples/bpf: Enable powerpc support")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://lore.kernel.org/r/20251020203643.989467-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>