riscv, bpf: Support sign-extension load insns

Add Support sign-extension load instructions for RV64.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Acked-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/r/20230824095001.3408573-3-pulehui@huaweicloud.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>