bpf, riscv: Support riscv jit to provide bpf_line_info

Add support for riscv jit to provide bpf_line_info. We need to
consider the prologue offset in ctx->offset, but unlike x86 and
arm64, ctx->offset of riscv does not provide an extra slot for
the prologue, so here we just calculate the len of prologue and
add it to ctx->offset at the end. Both RV64 and RV32 have been
tested.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220530092815.1112406-3-pulehui@huawei.com