riscv, bpf: Add kfunc support for RV64

riscv, bpf: Add kfunc support for RV64

This patch adds kernel function call support for RV64. Since the offset
from RV64 kernel and module functions to bpf programs is almost within
the range of s32, the current infrastructure of RV64 is already
sufficient for kfunc, so let's turn it on.

Suggested-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Acked-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20230221140656.3480496-1-pulehui@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>