perf build: Pre-generate BPF skeleton tooling during umbrella prepare phase

Currently, BPF skeleton generation rules (bpf_skel.mak) are evaluated as
part of util/Build. However, because LIBPERF_UTIL_IN explicitly depends
on the top-level static libbpf archive, Make completely blocked the
execution of bpftool bootstrap and skeleton generation until libbpf
finished compiling midway through the build.

Since bpftool bootstrap compiles its own independent copy of libbpf.a, it
does not depend on the top-level libbpf target.

Decouple early skeleton tooling generation by attaching bpf-skel-prepare
to the umbrella prepare target, exporting CONFIG_PERF_BPF_SKEL to ensure
accurate feature propagation. This allows Make to compile bpftool and
dump vmlinux.h in the background at build startup, eliminating the initial
sub-make startup bottleneck before BPF object compilation while keeping
100% of tooling rules perfectly encapsulated in bpf_skel.mak. Provide an
empty fallback target to ensure builds succeed when BPF skeletons are
disabled.

Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Gemini:gemini-3.1-pro-preview
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: James Clark <james.clark@linaro.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Chartre <alexandre.chartre@oracle.com>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>
Cc: Collin Funk <collin.funk1@gmail.com>
Cc: Costa Shulyupin <costa.shul@redhat.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Dapeng Mi <dapeng1.mi@linux.intel.com>
Cc: David Sterba <dsterba@suse.com>
Cc: Dmitrii Dolgov <9erthalion6@gmail.com>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Howard Chu <howardchu95@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Leo Yan <leo.yan@arm.com>
Cc: Markus Mayer <mmayer@broadcom.com>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Terrell <terrelln@fb.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <pjw@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Monnet <qmo@kernel.org>
Cc: Ricky Ringler <ricky.ringler@proton.me>
Cc: Song Liu <song@kernel.org>
Cc: Swapnil Sapkal <swapnil.sapkal@amd.com>
Cc: Thomas Falcon <thomas.falcon@intel.com>
Cc: Tomas Glozar <tglozar@redhat.com>
Cc: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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