libbpf: Complete LoongArch (loongarch) spec in bpf_tracing.h

Add PARM6 through PARM8 definitions. Add kernel docs link describing ABI
for LoongArch.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230120200914.3008030-10-andrii@kernel.org