kbuild: Fix objtool dependency for 'OBJECT_FILES_NON_STANDARD_<obj> := n'

"OBJECT_FILES_NON_STANDARD_vma.o := n" has a dependency bug.  When
objtool source is updated, the affected object doesn't get re-analyzed
by objtool.

Peter's new variable-sized jump label feature relies on objtool
rewriting the object file.  Otherwise the system can fail to boot.  That
effectively upgrades this minor dependency issue to a major bug.

The problem is that variables in prerequisites are expanded early,
during the read-in phase.  The '$(objtool_dep)' variable indirectly uses
'$@', which isn't yet available when the target prerequisites are
evaluated.

Use '.SECONDEXPANSION:' which causes '$(objtool_dep)' to be expanded in
a later phase, after the target-specific '$@' variable has been defined.

Fixes: b9ab5ebb ("objtool: Add CONFIG_STACK_VALIDATION option")
Fixes: ab325704

 ("jump_label, x86: Allow short NOPs")
Reported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>