bpftool: Allow explicitly skip llvm, libbfd and libcrypto dependencies

Introduce SKIP_LLVM, SKIP_LIBBFD, and SKIP_CRYPTO build flags that let
users build bpftool without these optional dependencies.

SKIP_LLVM=1 skips LLVM even when detected. SKIP_LIBBFD=1 prevents the
libbfd JIT disassembly fallback when LLVM is absent. Together, they
produce a bpftool with no disassembly support.

SKIP_CRYPTO=1 excludes sign.c and removes the -lcrypto link dependency.
Inline stubs in main.h return errors with a clear message if signing
functions are called at runtime.

Use BPFTOOL_WITHOUT_CRYPTO (not HAVE_LIBCRYPTO_SUPPORT) as the C
define, following the BPFTOOL_WITHOUT_SKELETONS naming convention for
bpftool-internal build config, leaving HAVE_LIBCRYPTO_SUPPORT free for
proper feature detection in the future.

All three flags are propagated through the selftests Makefile to bpftool
sub-builds.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260312-b4-bpftool_build-v2-1-4c9d57133644@meta.com

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