libm: pass CFLAGS to e500 fenv

As noted by Jason Woodward

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

libm/powerpc/e500/fpu/Makefile.arch

@@ -11,6 +11,8 @@ libm_ARCH_SRC:=$(wildcard $(libm_ARCH_fpu_DIR)/*.c)
 libm_ARCH_OBJ:=$(patsubst $(libm_ARCH_fpu_DIR)/%.c,$(libm_ARCH_fpu_OUT)/%.o,$(libm_ARCH_SRC))
 endif
 
+CFLAGS-libm/$(TARGET_ARCH)/$(TARGET_SUBARCH)/fpu/ := $(CFLAGS-libm)
+
 libm_ARCH_OBJS:=$(libm_ARCH_OBJ)
 
 ifeq ($(DOPIC),y)