kbuild: add 'private' to target-specific variables

Currently, Kbuild produces inconsistent results in some cases.

You can do an interesting experiment using the --shuffle option, which
is supported by GNU Make 4.4 or later.

Set CONFIG_KVM_INTEL=y and CONFIG_KVM_AMD=m (or vice versa), and repeat
incremental builds w/wo --shuffle=reverse.

$ make
[ snip ]
CC arch/x86/kvm/kvm-asm-offsets.s

$ make --shuffle=reverse
[ snip ]
CC [M] arch/x86/kvm/kvm-asm-offsets.s

$ make
[ snip ]
CC arch/x86/kvm/kvm-asm-offsets.s

arch/x86/kvm/kvm-asm-offsets.s is rebuilt every time w/wo the [M] marker.

arch/x86/kvm/kvm-asm-offsets.s is built as built-in when it is built as
a prerequisite of arch/x86/kvm/kvm-intel.o, which is built-in.

arch/x86/kvm/kvm-asm-offsets.s is built as modular when it is built as
a prerequisite of arch/x86/kvm/kvm-amd.o, which is a module.

Another odd example is single target builds.

When CONFIG_LKDTM=m, drivers/misc/lkdtm/rodata.o can be built as
built-in or modular, depending on how it is built.

$ make drivers/misc/lkdtm/lkdtm.o
[ snip ]
CC [M] drivers/misc/lkdtm/rodata.o

$ make drivers/misc/lkdtm/rodata.o
[ snip ]
CC drivers/misc/lkdtm/rodata.o

drivers/misc/lkdtm/rodata.o is built as modular when it is built as a
prerequisite of another, but built as built-in when it is a final
target.

The same thing happens to drivers/memory/emif-asm-offsets.s when
CONFIG_TI_EMIF_SRAM=m.

$ make drivers/memory/ti-emif-sram.o
[ snip ]
CC [M] drivers/memory/emif-asm-offsets.s

$ make drivers/memory/emif-asm-offsets.s
[ snip ]
CC drivers/memory/emif-asm-offsets.s

This is because the part-of-module=y flag defined for the modules is
inherited by its prerequisites.

Target-specific variables are likely intended only for local use.
This commit adds 'private' to them.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>