simple aarch64 bare metal 'Hello World' on qemu

Since I didn't write bare metal program on ARMv8/aarch64 before, I'll just do one.

1. starting from https://balau82.wordpress.com/2010/02/28/hello-world-for-bare-metal-arm-using-qemu/ seems a good idea
2. the toolchain is different, we need aarch64 cross compile toolchain. I think both linux and bare metal toolchains work for my testing purpose. However, I'll use bare metal one from linaro (http://releases.linaro.org/14.11/components/toolchain/binaries/aarch64-none-elf/gcc-linaro-4.9-2014.11-x86_64_aarch64-elf.tar.xz)
3. startup.s needs a bit tweek. We cannot ldr a value to the stack pointer (sp) directly. So I load the starting address of stack to x30, then load x30 into sp.
4. my modified startup.s is named startup64.s which is compiled into startup64.o by 'aarch64-none-elf-as -g startup64.s -o startup64.o'
5. One of the working aarch64 targets of qemu is virt, a look into qemu's hw/arm/virt.c shows that the memory mapped UART0 is located at 0x0900000. So the UART0DR in test.c should be changed to 0x09000000. My modified test.c is called test64.c, which is compiled into .o by 'aarch64-none-elf-gcc -c -g test64.c -o test64.o'
6. The address to load the program to should be changed too. '0x10000' is not valid RAM or ROM address for -M virt. According to virt.c, '0x40000000' is OK to use. So my linker script is modified and named test64.ld
7. I use 'aarch64-none-elf-ld -T test64.ld test64.o startup64.o -o test64.elf' to generate test64.elf
8. That's it. Since qemu can load elf directly now. We can use 'qemu-system-aarch64 -M virt -cpu cortex-a57 -nographic -kernel test64.elf' to run the elf and see 'Hello Wolrd!'.