riscv, libbpf: Add RISC-V (RV64) support to bpf_tracing.h

Add macros for 64-bit RISC-V PT_REGS to bpf_tracing.h.

Signed-off-by: Björn Töpel <bjorn@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211028161057.520552-4-bjorn@kernel.org