"This AMI includes a set of developer tools that you can use in the AWS Cloud at no charge. You write your FPGA code using VHDL or Verilog and then compile, simulate, and verify it using tools from the Xilinx Vivado Design Suite (you can also use third-party simulators, higher-level language compilers, graphical programming tools, and FPGA IP libraries)."

So basically, buying a copy of Vivado is the minimum. There aren't any open source tools that directly output Xilinx FPGA bitstreams that I know of.