volk: force use of generic kernel when VOLK_GENERIC env variable is defined.

cmake: insert VOLK_GENERIC into QA codes to only test generic versions of kernels. Helps control precision issues.
volk: adds command-line option to enable/disable use of ORC with -DENABLE_ORC=True/False.

If ORC is found on the system, the default is to use it. If we have ORC but don't want to use it, we can turn it off using this.