Zvbb Proof of Concept

The zvbb-test.c file contains a set of functions verifying and
showcasing usage of instructions defined by the Zvbb extension.

Pre-requisites:

- riscv64-linux-gnu toolchain available in PATH.
- Zvbb-enabled binutils-gdb available in PATH overriding the above
  toolchain.
- Zvbb-enabled Spike available in PATH.
- RISC-V Proxy kernel (riscv-pk) compiled and available in
  ~/RISC-V/riscv64-linux-gnu/bin/pk (can be overridden with PK make
  variable).

The default make target will compile all examples and run them in Spike
simulator.

Example make invocation:
    # Build and run Zvbb tests with default parameters
    make run-zvbb
    # Override riscv-pk location
    make run-zvbb PK=/opt/prefix/riscv64-linux-gnu/bin/pk
    # Override target triplet and riscv-pk location
    make run-zvbb TARGET=riscv64-unknown-linux-gnu \
                  PK=/opt/prefix/riscv64-linux-gnu/bin/pk

Make targets:
- zvbb-test - compiles the PoC code.
- run-zvbb - Executes the compiled binary in Spike.
- clean - Cleans the build artifacts.

Make variables:
- TARGET - Target triplet to use. By default riscv64-linux-gnu.
- PK - Location of the riscv-pk binary. By default it's
  ~/RISC-V/$(TARGET)/bin/pk

See Makefile for more details.

Signed-off-by: Stanislaw Kardach <kda@semihalf.com>
Signed-off-by: Eric Gouriou <ego@rivosinc.com>