kbuild,objtool: Defer objtool validation step for CONFIG_KLP_BUILD

kbuild,objtool: Defer objtool validation step for CONFIG_KLP_BUILD

In preparation for klp-build, defer objtool validation for
CONFIG_KLP_BUILD kernels until the final pre-link archive (e.g.,
vmlinux.o, module-foo.o) is built.  This will simplify the process of
generating livepatch modules.

Delayed objtool is generally preferred anyway, and is already standard
for IBT and LTO.  Eventually the per-translation-unit mode will be
phased out.

Acked-by: Petr Mladek <pmladek@suse.com>
Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>