riscv, bpf: Add support for far branching

This commit adds branch relaxation to the BPF JIT, and with that
support for far (offset greater than 12b) branching.

The branch relaxation requires more than two passes to converge. For
most programs it is three passes, but for larger programs it can be
more.

Signed-off-by: Björn Töpel <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Reviewed-by: Luke Nelson <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]