objtool/klp: Add .klp.symid for sympos disambiguation

Livepatch identifies a duplicate-named symbol by its position (sympos) among same-named kallsyms entries, which for vmlinux are counted in ascending address order in the final linked kernel. That order can't be reliably derived from vmlinux.o: the final link reorders sub-sections (.text.unlikely*, .data..*, etc). Bridge the gap with a new .klp.symid section which can be used to correlate symbols between vmlinux.o and vmlinux so that klp-diff can reliably determine the sympos. The table can't survive --gc-sections: keeping it alive would keep every duplicate-named symbol's section alive, so the reference kernel would stop matching the one which ships. klp-build rejects CONFIG_LD_DEAD_CODE_DATA_ELIMINATION instead. Nothing is lost today: x86_64 is the only HAVE_KLP_BUILD arch and doesn't select HAVE_LD_DEAD_CODE_DATA_ELIMINATION, arm64 and s390 have never selected it either, and on powerpc, it's still EXPERIMENTAL and disabled by every distro kernel. This is the build-time half of reliable vmlinux sympos computation; "objtool klp diff" will consume the table in a subsequent commit.

JPJosh Poimboeuf committed 3 months ago