kconfig.h: use already defined macros for IS_REACHABLE() define

For the same reason as commit 02d699f1

 ("include/linux/kconfig.h:
ese macros which are already defined"), it is better to use macros
IS_BUILTIN() and IS_MODULE() for defining IS_REACHABLE().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Marek <mmarek@suse.com>