kati doesn't support .SECONDEXPANSION

[DelilahHoare]

The .SECONDEXPANSION special target does not have any effect in kati other than to print a warning that it is not supported.

This feature is used heavily by Linux's build system. The rule $(obj)/%.o: $(src)/%.c ... in scripts/Makefile.build also takes $$(objtool_dep) as an input, which without secondary expansion will be treated as the literal file $(objtool_dep), which does not exist and cannot be made, so C files are instead compiled with the builtin rule, which doesn't work.