sparc: fix KBUILD_DEFCONFIG for ARCH=sparc32

As commit 5ba80096 ("kbuild: update ARCH alias info for sparc")
addressed, SPARC accepts ARCH=sparc32 as an alias.

However, arch/sparc/Makefile wrongly sets KBUILD_DEFCONFIG, then
sparc64_defconfig is chosen as the base configuration for ARCH=sparc32.

  $ make ARCH=sparc32 defconfig
  *** Default configuration is based on 'sparc64_defconfig'
  #
  # configuration written to .config
  #

Fix the logic to choose sparc64_defconfig only when ARCH=sparc64.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: David S. Miller <davem@davemloft.net>