libbpf: Define powerpc syscall regs spec in bpf_tracing.h

Define explicit table of registers used for syscall argument passing.
Note that 7th arg is supported on 32-bit powerpc architecture, by not on
powerpc64.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230120200914.3008030-20-andrii@kernel.org