libbpf: Complete sparc spec in bpf_tracing.h

Add PARM6 definition for sparc architecture. Leave a link to calling
convention documentation.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230120200914.3008030-7-andrii@kernel.org