riscv: bpf: Fix eBPF's exception tables

eBPF's exception tables needs to be modified to relative synchronously.

Suggested-by: Tong Tiangen <tongtiangen@huawei.com>
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Fixes: 1f77ed94 ("riscv: switch to relative extable and other improvements")
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>