Use cilium 0.20 because of regression due to cilium/ebpf#1882

Cleanups based on integrating with parca-dev/opentelemetry-ebpf-profi…

…ler (#3)

1. Renamed ebpf/usdt.h → ebpf/usdt_defs.h (via git mv).
  2. ebpf/usdt_defs.h: removed #include "kernel.h", updated include guard to USDT_USDT_DEFS_H, added a
  comment block explaining that the file deliberately includes nothing and consumers must provide the
  integer typedefs.
  3. ebpf/usdt_args.h: changed #include "usdt.h" → #include "usdt_defs.h". Still doesn't include
  kernel.h — consumers include their prelude before this header.
  4. ebpf/usdt.ebpf.c: now includes "kernel.h" then "usdt_defs.h" (was just "usdt.h", which transitively
   pulled kernel.h).
  5. ebpf/usdt_test.ebpf.c: added #include "kernel.h" before #include "usdt_args.h" for the same reason.
  6. types_def.go: cgo preamble now includes <stdint.h>/<stdbool.h> and locally typedefs u8..u64/s8..s64
   before including ebpf/usdt_defs.h (since the header no longer pulls in kernel.h). Comments updated to
   reference usdt_defs.h. types.go regenerated identically (struct layouts unchanged).
  7. README.md: updated the BPF example to show #include "kernel.h" before #include "usdt_args.h", with
  a note that usdt_defs.h is the standalone option for userspace-visible layouts.
  8. bpfusdt_x86_bpfel.o / bpfusdt_arm64_bpfel.o: regenerated by go generate for both architectures.
  Tests pass: ok github.com/parca-dev/usdt, ok github.com/parca-dev/usdt/test.