Hello world

This demo prints a Hello world string to the JTAG semihosting interface.

How to run this demo:

Build and load a IBEX bitstream
Build and load the demo with cargo run.
Connect openocd
Run the gdb:

$ riscv32-unknown-elf-gdb ../../target/riscv32imc-unknown-none-elf/debug/led

(gdb) # Connect to OpenOCD
(gdb) target remote :3333

(gdb) # Enable OpenOCD's semihosting support
(gdb) monitor arm semihosting enable

(gdb) # Flash the program
(gdb) load

(gdb) # Run the program
(gdb) continue

The log will show up on the openocd console (Step 3).