lib: libc: Build with -fno-builtin

Without -fno-builtin the compiler might attempt to optimize the libc
code by replacing loops with calls to libc code, potentially causing
infinite loops. This seems to cause problems on some platforms with
GCC 12.

lib/libc/rules.mk

@@ -10,6 +10,9 @@ OBJS += \
 	$(LOCAL_DIR)/rand.o \
 	$(LOCAL_DIR)/eabi.o
 
+# Without -fno-builtin the compiler might attempt to optimize the libc code by
+# replacing loops with calls to libc code, potentially causing infinite loops.
+$(BUILDDIR)/$(LOCAL_DIR)/%.o: CFLAGS := $(CFLAGS) -fno-builtin
 
 include $(LOCAL_DIR)/string/rules.mk