um: stop abusing KBUILD_KCONFIG

Instead create a arch/um/Kconfig file that just includes the actual
per-arch Kconfig file.  Note that we use HEADER_ARCH to find the
per-arch Kconfig file as that variable already includes the
normalization from i386 or x86_64 to x86.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>