Write a C compiler with Ziglang

Compile an integer to an exectuable that exit with the given number

$ cd zcc
$ zig build run -- 10
$ gcc -static tmp.s -o tmp
$ ./tmp
$ echo $?
  10

Reference

chibicc: 0522e2d

Add + and - operators

$ cd zcc
$ zig build run -- 5+20-4
$ gcc -static tmp.s -p tmp
$ ./tmp
$ echo $?
  21

Reference

chibicc: bf7081f