bpf_cflags doesn't contain '-target bpf' because SkeletonBuilder does so
internally. However, bindgen doesn't and we end up specifying options which
are specific to bpf target without actually specifying the target, this
leads to warnings like the followings:
[scx_layered 0.0.1] warning: unknown warning option '-Wno-compare-distinct-pointer-types''; did you mean '-Wno-compare-distinct-pointer-types'? [-Wunknown-warning-option]
[scx_layered 0.0.1] clang diag: warning: argument unused during compilation: '-mcpu=v3' [-Wunused-command-line-argument]
[scx_layered 0.0.1] clang diag: warning: unknown warning option '-Wno-compare-distinct-pointer-types''; did you mean '-Wno-compare-distinct-pointer-types'? [-Wunknown-warning-option]
Fix it by adding '-target bpf' when invoking bindgen.
