libbpf: Complete riscv arch spec in bpf_tracing.h

Add PARM6 through PARM8 definitions for RISC V (riscv) arch. Leave the
link for ABI doc for future reference.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Pu Lehui <pulehui@huawei.com> # RISC-V
Link: https://lore.kernel.org/bpf/20230120200914.3008030-8-andrii@kernel.org