I write a program byte by byte. This helps us understand how ELF is constructed and the way to relocate outer symbols.
main.c: Basic ELF construction, is only an empty program return 0.
hello_world.c: Advanced to relocating symbol such as printf by linker.
arm-linux-gnueabihf-gcc
qemu-arm
arm-linux-gnueabihf library
http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html
https://github.com/mydzor/tinyelf-arm