Add option to select Toolchain from command-line

Hi @strichmo, as discussed here is an attempt to allow users to select their toolchain and maintain backward compatibility for Contributors that do not want/need to change:

From the comment block in core-v-verif/cv32/sim/Common.mk:

# "Toolchain" to compile 'test-programs' (either C or RISC-V Assember) for the
# CV32E40P.   This toolchain is used by both the core testbench and UVM
# environment.  The assumption here is that you have installed at least one of
# the following toolchains:
#     1. GNU:   https://github.com/riscv/riscv-gnu-toolchain
#               Assumed to be installed at /opt/gnu.
#
#     2. COREV: https://www.embecosm.com/resources/tool-chain-downloads/#corev 
#               Assumed to be installed at /opt/corev.
#
#     3. PULP:  https://github.com/pulp-platform/pulp-riscv-gnu-toolchain 
#               Assumed to be installed at /opt/pulp.
#
# If you do not select one of the above options, compilation will be attempted
# using whatever is found at /opt/riscv using arch=unknown.
#

Signed-off-by: Mike Thompson mike@openhwgroup.org