Use string type with -release compilation flag to avoid undefined reference to __assert

Makefile

@@ -1,7 +1,7 @@
 .PHONY: kernel
 kernel:
 	./i686-elf-gcc/bin/i686-elf-gcc -c -o boot.o -ffreestanding boot.S
-	ldc2 -march=x86 -mcpu=i686 --betterC -c -of=kernel_main.o kernel_main.d
+	ldc2 -march=x86 -mcpu=i686 --betterC -release -c -of=kernel_main.o kernel_main.d
 	./i686-elf-gcc/bin/i686-elf-gcc -o kernel.elf -ffreestanding -nostdlib -T link.ld boot.o kernel_main.o
 
 .PHONY: image