libbpf, riscv: Use a0 for RC register

According to the RISC-V calling convention register usage here [0], a0
is used as return value register, so rename it to make it consistent
with the spec.

  [0] section 18.2, table 18.2
      https://riscv.org/wp-content/uploads/2015/01/riscv-calling.pdf



Fixes: 589fed47 ("riscv, libbpf: Add RISC-V (RV64) support to bpf_tracing.h")
Signed-off-by: Yixun Lan <dlan@gentoo.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Amjad OULED-AMEUR <ouledameur.amjad@gmail.com>
Link: https://lore.kernel.org/bpf/20220706140204.47926-1-dlan@gentoo.org