This adds a QEMU-based bare metal RISC-V 32 bits port. For the time being
only QEMU's "virt" 32 bits board is supported, using the ilp32 ABI and the
RV32IMC architecture.
The top-level README and the run-tests.py files are updated for this new
port.
Signed-off-by: Alessandro Gatti <a.gatti@frob.it>