blackfin: Move `-D__BLACKFIN__' to $(ARCH)_config.mk

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>
diff --git a/blackfin_config.mk b/blackfin_config.mk
index df324b7..f71a313 100644
--- a/blackfin_config.mk
+++ b/blackfin_config.mk
@@ -21,4 +21,4 @@
 # MA 02111-1307 USA
 #
 
-PLATFORM_CPPFLAGS += -DCONFIG_BLACKFIN
+PLATFORM_CPPFLAGS += -DCONFIG_BLACKFIN -D__BLACKFIN__