Refactoring the method for selecting toolchain
This pull-request implements a prototype of a scheme to refactor how the core-v-verif Makefiles select toolchains. User-level documentation is available in mk/uvmt/README.md and implementer-level documentation is the common Makefile, (mk/Common.mk here and here).
The original code in the Makefile has not yet been deleted, just commented out.
These changes successfully passed a local "ci_check" regression with dsim.
Signed-off-by: Mike Thompson mike@openhwgroup.org