bpf: register compile_commands.json entries for bpf programs

bpf: register compile_commands.json entries for bpf programs

compile_commands.json is generated by ninja from c_COMPILER rules, so
BPF programs (built via custom_target and thus emitted as CUSTOM_COMMAND
rules) never show up there. Clangd consequently falls back to
heuristics when opening .bpf.c files, with poor diagnostic fidelity.

Register a meson postconf script per BPF program that upserts an entry
into compile_commands.json using the same argv meson constructed for
the custom_target. The script runs after meson has written the DB,
substitutes @INPUT@/@OUTPUT@, and keys entries by source path so
repeated reconfigures don't accumulate duplicates.