mips: bpf: implement jitting of BPF_ALU | BPF_ARSH | BPF_X

Jitting of BPF_K is supported already, but not BPF_X. This patch complete
the support for the latter on both MIPS and microMIPS.

Cc: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Acked-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>