chore: drop -fno-builtin

It is implied by -ffreestanding

Change-Id: Ia7b0f78fd4020aceea246d10ac3d869e743e7a9f
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
diff --git a/Makefile b/Makefile
index 6e0e584..6f080b4 100644
--- a/Makefile
+++ b/Makefile
@@ -283,7 +283,7 @@
 				-ffreestanding -Wa,--fatal-warnings
 TF_CFLAGS		+=	$(CPPFLAGS) $(TF_CFLAGS_$(ARCH))		\
 				-ffunction-sections -fdata-sections		\
-				-ffreestanding -fno-builtin -fno-common		\
+				-ffreestanding -fno-common			\
 				-Os -std=gnu99
 
 ifeq (${SANITIZE_UB},on)