move -ffixed-P5 to blackfin_config.mk and drop unused -D__BLACKFIN__

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/cpu/bf537/config.mk b/cpu/bf537/config.mk
index 8a35789..fbbe75d 100644
--- a/cpu/bf537/config.mk
+++ b/cpu/bf537/config.mk
@@ -24,4 +24,4 @@
 # MA 02110-1301 USA
 #
 
-PLATFORM_RELFLAGS += -mcpu=bf537 -ffixed-P5
+PLATFORM_RELFLAGS += -mcpu=bf537