objtool/klp: Check the klp test environment once, before any test

Every test checked for itself that objtool exists, was built with klp
support, and that $CC runs.  Three problems with that: it is the same work
done 40 times, a missing objtool reads as a per-test skip rather than as a
suite which cannot run, and a run in which everything skipped still exits
0.

Do it once, before any test, in klp_preflight() in lib.sh, and export the
answers where the tests can read them.  If the suite cannot run the whole
run fails and says why; a test which gets as far as running can assume its
environment.

Nothing is written down, so nothing can go stale.  Sourcing lib.sh runs the
checks, which means a test run by hand establishes its own answers rather
than inheriting those of some earlier run with a different CC, and a test
is never handed an empty value for where objtool is or which architecture
it is on: either the exports are set or the run has already failed.

preflight answers only whether the suite can run at all -- not what the
compiler is capable of.  A test needing a particular compiler feature
probes for it and skips; that costs one compile and keeps the reason next
to the test that has to justify it.

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/20260916184351.2720310-3-song@kernel.org

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