riscv, bpf: Add ex_insn_off and ex_jmp_off for exception table handling

Add ex_insn_off and ex_jmp_off fields to struct rv_jit_context so that
add_exception_handler() does not need to be immediately followed by the
instruction to add the exception table. ex_insn_off indicates the offset
of the instruction to add the exception table, and ex_jmp_off indicates
the offset to jump over the faulting instruction. This is to prepare for
adding the exception table to atomic instructions later, because some
atomic instructions need to perform zext or other operations.

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/20250719091730.2660197-9-pulehui@huaweicloud.com