libbpf: Fix comment about arc and riscv arch in bpf_tracing.h

To make comments about arc and riscv arch in bpf_tracing.h accurate,
this patch fixes the comment about arc and adds the comment for riscv.

Signed-off-by: Kenjiro Nakayama <nakayamakenjiro@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230504035443.427927-1-nakayamakenjiro@gmail.com