objtool/klp: Add test for absolute and __ADDRESSABLE symbols

A SHN_ABS symbol has no section, so any walk of sym->sec which does not
check dereferences NULL, and the kernel has plenty of them -- from linker
scripts and from .set in assembly.  __ADDRESSABLE() emits a pointer into
.discard.addressable purely to keep a symbol referenced; it means nothing
to a livepatch and is discarded at link time, but it is a relocation like
any other and gets looked at.

Neither is what the patch changes.  What this guards against is not a wrong
answer but a crash or an error on input the kernel produces routinely,
which would make every function near one unpatchable.

Not isolated to a single line, and the test says so: the absolute symbol
here has zero length, so it is excluded before the section check is reached
and removing that check alone changes nothing observable.

Assisted-by: Claude:claude-opus-4
Based-on-test-by: Joe Lawrence <joe.lawrence@redhat.com>
Assisted-by: Claude:claude-opus-5
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://patch.msgid.link/20260916184351.2720310-56-song@kernel.org

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