nfp: bpf: implement jitting of BPF_ALU | BPF_ARSH | BPF_*

BPF_X support needs indirect shift mode, please see code comments for
details.

Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>