kbuild: replace $(linked-object) with CONFIG options

*.prelink.o is created when CONFIG_LTO_CLANG or CONFIG_X86_KERNEL_IBT
is enabled.

Replace $(linked-object) with $(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT)
so you will get a quick idea of when the --link option is passed.

No functional change is intended.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM-14