riscv, bpf: Add support arena atomics for RV64

Add arena atomics support for RMW atomics and load-acquire and
store-release instructions. Non-Zacas cmpxchg is implemented via loop,
which is not currently supported because it requires more complex
extable and loop logic.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Acked-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/bpf/20250719091730.2660197-10-pulehui@huaweicloud.com