objtool: Fix init_module() handling

If IBT is enabled and a module uses the deprecated init_module() magic
function name rather than module_init(fn), its ENDBR will get removed,
causing an IBT failure during module load.

Objtool does print an obscure warning, but then does nothing to either
correct it or return an error.

Improve the usefulness of the warning and return an error so it will at
least fail the build with CONFIG_OBJTOOL_WERROR.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/366bfdbe92736cde9fb01d5d3eb9b98e9070a1ec.1742852846.git.jpoimboe@kernel.org