bpftool: Restrict feature tests during bootstrap compilation

When the perf build executes 'make -C ../bpf/bpftool bootstrap', bpftool's
Makefile unconditionally evaluated feature checks for llvm, libcap, libbfd,
and disassembler libraries because the bootstrap target was not exempted.

Since the bootstrap bpftool strictly compiles minimal AST parsing and C
code generation logic without linking LLVM or disassembler libraries, these
feature check sub-makes are completely redundant.

Exempt the bootstrap target from non-essential feature tests to eliminate
unneeded sub-make fork overhead during Kbuild startup.

Tested-by: James Clark <james.clark@linaro.org>
Assisted-by: Gemini:gemini-3.1-pro-preview
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/r/20260531010750.525160-1-irogers@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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