fix: restore builds without libbpf

[yunwei37]

Summary

• remove unconditional libbpf headers and kernel-program operations from the no-libbpf runtime path
• use a direct Linux BPF_OBJ_GET_INFO_BY_FD syscall fallback and return ENOTSUP on non-Linux no-libbpf platforms
• skip libbpf-dependent AOT/LLVM CLI targets while retaining the LLVM JIT runtime
• use the configured LLVM library directories instead of a hardcoded LLVM 20 path
• add a GCC 13 compile smoke build for BPFTIME_BUILD_WITH_LIBBPF=OFF

Validation

• fresh BPFTIME_BUILD_WITH_LIBBPF=OFF, BPFTIME_LLVM_JIT=ON configure and full build
• confirmed the no-libbpf build does not build or download libbpf sources
• bpftime_runtime_tests compiled with the normal libbpf-enabled configuration
• [queue_map]: 289 assertions passed
• [stack_map]: 310 assertions passed
• [bloom_filter]: 32,111 assertions passed
• git diff --check
• dedicated and independent read-only reviews found no blocking issues

Fixes #603

[yunwei37]

The integrated-example failure was infrastructure-related: the Fedora 39 base-image pull timed out three times before checkout, so no repository code had run. I reran the failed jobs on the same head, and the full 120-job integrated-example fan-out is now green.

The no-libbpf smoke build and the remaining standard checks are also green. The two GPU jobs waited for self-hosted runners for 24 hours and were then cancelled without running any steps:

• build-bpftime-gpu
• test-ptxpass

There are no unresolved review threads. The remaining gates are GPU runner availability and maintainer review.

AI-generated response; a maintainer will review and follow up later