kernel: enable CONFIG_BPF_JIT by default

kernel: enable CONFIG_BPF_JIT by default

Enable the built-in BPF JIT compiler for all 4.9, 4.14 and 4.19 kernels,
which should speed up cBPF and eBPF-based packet filtering (tc, iptables)
and packet sniffing (libpcap, tcpdump, fwknopd, etc).

This has minimal kernel size impact, increasing the size of uImage-lzma
(normally ~2 MB on mips_24kc or mips64el_mips64) by 5 KB for the MIPS32
arch cBPF JIT and by 9 KB for the MIPS64 arch eBPF JIT, on kernel 4.14.

With JIT enabled (cBPF only), the standard BPF test module (test_bpf.ko)
running on a DIR-835 (mips_24kc) used 33 CPU seconds, but 68 without JIT.

This change aligns with the notion of OpenWRT as the network go-to swiss
army knife for packet handling, especially on CPU-constrained platforms.

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>