sh: select KBUILD_DEFCONFIG depending on ARCH

You can not select KBUILD_DEFCONFIG depending on any CONFIG option
because include/config/auto.conf is not included when building config
targets.  So, CONFIG_SUPERH32 is never set during the configuration,
then cayman_defconfig is always chosen.

This commit provides a sensible way to choose shx3/cayman_defconfig.

arch/sh/Kconfig sets either SUPERH32 or SUPERH64 depending on ARCH
environment, like follows:

  config SUPERH32
          def_bool ARCH = "sh"

          ...

  config SUPERH64
          def_bool ARCH = "sh64"

It should make sense to choose the default defconfig by ARCH,
like arch/sparc/Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>