objtool: Add test harness for the klp subcommands

"objtool klp checksum" and "objtool klp diff" work on object files alone,
with no kernel, vmlinux or configuration involved, so they can be tested
directly. That is worth doing: most klp generation bugs so far have been in
symbol correlation, special section extraction and relocation conversion,
and several of them failed silently, producing a livepatch which built
cleanly but was missing data.

A test compiles a fixture twice, as the original and (with -DPATCHED) the
patched object, runs both through klp checksum, diffs them and asserts on
the result.  Fixtures are compiled at test time rather than committed as
binaries: codegen varies between compilers and architectures, and that
variation is where a good number of these bugs come from.

Assertions check properties rather than compare against recorded output.
Golden files would need re-recording for every compiler change and would
report churn instead of regressions.

klp diff resolves symbols against Module.symvers, so the harness writes
one. Whether a symbol is listed there decides between an ordinary
relocation and a klp relocation, which makes it the main knob tests use.

Run with:

  make -C tools/objtool tests

Tests skip when objtool was built without klp support or when a fixture
does not build for the target architecture.  A missing objtool binary fails
instead of skipping, since that means a broken invocation rather than an
environment which cannot run the test.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Co-developed-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Assisted-by: Claude:claude-opus-5
Link: https://patch.msgid.link/20260916184351.2720310-2-song@kernel.org

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